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L_0723 | what is force | T_3571 | Force is a vector because it has both size and direction. For example, the girl in Figure 13.1 is pushing the swing away from herself. Thats the direction of the force. She can give the swing a strong push or a weak push. Thats the size, or strength, of the force. Like other vectors, forces can be represented with arrows. Figure 13.2 shows some examples. The length of each arrow represents the strength of the force, and the way the arrow points represents the direction of the force. How could you use an arrow to represent the girls push on the swing in Figure 13.1? | text | null |
L_0723 | what is force | T_3571 | Force is a vector because it has both size and direction. For example, the girl in Figure 13.1 is pushing the swing away from herself. Thats the direction of the force. She can give the swing a strong push or a weak push. Thats the size, or strength, of the force. Like other vectors, forces can be represented with arrows. Figure 13.2 shows some examples. The length of each arrow represents the strength of the force, and the way the arrow points represents the direction of the force. How could you use an arrow to represent the girls push on the swing in Figure 13.1? | text | null |
L_0723 | what is force | T_3572 | The SI unit of force is the newton (N). One newton is the amount of force that causes a mass of 1 kilogram to accelerate at 1 m/s2 . Thus, the newton can also be expressed as kgm/s2 . The newton was named for the scientist Sir Isaac Newton, who is famous for his law of gravity. Youll learn more about Sir Isaac Newton later in the chapter. | text | null |
L_0723 | what is force | T_3573 | More than one force may act on an object at the same time. In fact, just about all objects on Earth have at least two forces acting on them at all times. One force is gravity, which pulls objects down toward the center of Earth. The other force is an upward force that may be provided by the ground or other surface. Consider the example in Figure 13.3. A book is resting on a table. Gravity pulls the book downward with a force of 20 newtons. At the same time, the table pushes the book upward with a force of 20 newtons. The combined forces acting on the book or any other object are called the net force. This is the overall force acting on an object that takes into account all of the individual forces acting on the object. You can learn more about the concept of net force at this URL: . | text | null |
L_0723 | what is force | T_3574 | When two forces act on an object in opposite directions, like the book on the table, the net force is equal to the difference between the two forces. In other words, one force is subtracted from the other to calculate the net force. If the opposing forces are equal in strength, the net force is zero. Thats what happens with the book on the table. The upward force minus the downward force equals zero (20 N up - 20 N down = 0 N). Because the forces on the book are balanced, the book remains on the table and doesnt move. In addition to these downward and upward forces, which generally cancel each other out, forces may push or pull an object in other directions. Look at the dogs playing tug-of-war in Figure 13.4. One dog is pulling on the rope with a force of 10 newtons to the left. The other dog is pulling on the rope with a force of 12 newtons to the right. These opposing forces are not equal in strength, so they are unbalanced. When opposing forces are unbalanced, the net force is greater than zero. The net force on the rope is 2 newtons to the right, so the rope will move to the right. | text | null |
L_0723 | what is force | T_3575 | Two forces may act on an object in the same direction. You can see an example of this in Figure 13.5. After the man on the left lifts up the couch, he will push the couch to the right with a force of 25 newtons. At the same time, the man to the right is pulling the couch to the right with a force of 20 newtons. When two forces act in the same direction, the net force is equal to the sum of the forces. This always results in a stronger force than either of the individual forces alone. In this case, the net force on the couch is 45 newtons to the right, so the couch will move to the right. You Try It! Problem: The boys in the drawing above are about to kick the soccer ball in opposite directions. What will be the net force on the ball? In which direction will the ball move? | text | null |
L_0723 | what is force | T_3575 | Two forces may act on an object in the same direction. You can see an example of this in Figure 13.5. After the man on the left lifts up the couch, he will push the couch to the right with a force of 25 newtons. At the same time, the man to the right is pulling the couch to the right with a force of 20 newtons. When two forces act in the same direction, the net force is equal to the sum of the forces. This always results in a stronger force than either of the individual forces alone. In this case, the net force on the couch is 45 newtons to the right, so the couch will move to the right. You Try It! Problem: The boys in the drawing above are about to kick the soccer ball in opposite directions. What will be the net force on the ball? In which direction will the ball move? | text | null |
L_0724 | friction | T_3576 | Friction is a force that opposes motion between two surfaces that are touching. Friction can work for or against us. For example, putting sand on an icy sidewalk increases friction so you are less likely to slip. On the other hand, too much friction between moving parts in a car engine can cause the parts to wear out. Other examples of friction are illustrated in Figure 13.7. You can see an animation showing how friction opposes motion at this URL: http://w | text | null |
L_0724 | friction | T_3577 | Friction occurs because no surface is perfectly smooth. Even surfaces that look smooth to the unaided eye appear rough or bumpy when viewed under a microscope. Look at the metal surfaces in Figure 13.8. The metal foil is so smooth that it is shiny. However, when highly magnified, the surface of metal appears to be very bumpy. All those mountains and valleys catch and grab the mountains and valleys of any other surface that contacts the metal. This creates friction. | text | null |
L_0724 | friction | T_3578 | Rougher surfaces have more friction between them than smoother surfaces. Thats why we put sand on icy sidewalks and roads. The blades of skates are much smoother than the soles of shoes. Thats why you cant slide as far across ice with shoes as you can with skates (see Figure 13.9). The rougher surface of shoes causes more friction and slows you down. Heavier objects also have more friction because they press together with greater force. Did you ever try to push boxes or furniture across the floor? Its harder to overcome friction between heavier objects and the floor than it is between lighter objects and the floor. | text | null |
L_0724 | friction | T_3579 | You know that friction produces heat. Thats why rubbing your hands together makes them warmer. But do you know why the rubbing produces heat? Friction causes the molecules on rubbing surfaces to move faster, so they have more heat energy. Heat from friction can be useful. It not only warms your hands. It also lets you light a match (see Figure 13.10). On the other hand, heat from friction can be a problem inside a car engine. It can cause the car to overheat. To reduce friction, oil is added to the engine. Oil coats the surfaces of moving parts and makes them slippery so there is less friction. | text | null |
L_0724 | friction | T_3580 | There are different ways you could move heavy boxes. You could pick them up and carry them. You could slide them across the floor. Or you could put them on a dolly like the one in Figure 13.11 and roll them across the floor. This example illustrates three types of friction: static friction, sliding friction, and rolling friction. Another type of friction is fluid friction. All four types of friction are described below. In each type, friction works opposite the direction of the force applied to a move an object. You can see a video demonstration of the different types of friction at this URL: (1:07). | text | null |
L_0724 | friction | T_3581 | Static friction acts on objects when they are resting on a surface. For example, if you are walking on a sidewalk, there is static friction between your shoes and the concrete each time you put down your foot (see Figure 13.12). Without this static friction, your feet would slip out from under you, making it difficult to walk. Static friction also allows you to sit in a chair without sliding to the floor. Can you think of other examples of static friction? | text | null |
L_0724 | friction | T_3582 | Sliding friction is friction that acts on objects when they are sliding over a surface. Sliding friction is weaker than static friction. Thats why its easier to slide a piece of furniture over the floor after you start it moving than it is to get it moving in the first place. Sliding friction can be useful. For example, you use sliding friction when you write with a pencil and when you put on your bikes brakes. | text | null |
L_0724 | friction | T_3583 | Rolling friction is friction that acts on objects when they are rolling over a surface. Rolling friction is much weaker than sliding friction or static friction. This explains why it is much easier to move boxes on a wheeled dolly than by carrying or sliding them. It also explains why most forms of ground transportation use wheels, including cars, 4-wheelers, bicycles, roller skates, and skateboards. Ball bearings are another use of rolling friction (see Figure | text | null |
L_0724 | friction | T_3584 | Fluid friction is friction that acts on objects that are moving through a fluid. A fluid is a substance that can flow and take the shape of its container. Fluids include liquids and gases. If youve ever tried to push your open hand through the water in a tub or pool, then youve experienced fluid friction between your hand and the water. When a skydiver is falling toward Earth with a parachute, fluid friction between the parachute and the air slows the descent (see Figure 13.14). Fluid pressure with the air is called air resistance. The faster or larger a moving object is, the greater is the fluid friction resisting its motion. The very large surface area of a parachute, for example, has greater air resistance than a skydivers body. | text | null |
L_0725 | gravity | T_3585 | Gravity has traditionally been defined as a force of attraction between two masses. According to this conception of gravity, anything that has mass, no matter how small, exerts gravity on other matter. The effect of gravity is that objects exert a pull on other objects. Unlike friction, which acts only between objects that are touching, gravity also acts between objects that are not touching. In fact, gravity can act over very long distances. | text | null |
L_0725 | gravity | T_3586 | You are already very familiar with Earths gravity. It constantly pulls you toward the center of the planet. It prevents you and everything else on Earth from being flung out into space as the planet spins on its axis. It also pulls objects above the surface, from meteors to skydivers, down to the ground. Gravity between Earth and the moon and between Earth and artificial satellites keeps all these objects circling around Earth. Gravity also keeps Earth moving around the sun. | text | null |
L_0725 | gravity | T_3587 | Weight measures the force of gravity pulling on an object. Because weight measures force, the SI unit for weight is the newton (N). On Earth, a mass of 1 kilogram has a weight of about 10 newtons because of the pull of Earths gravity On the moon, which has less gravity, the same mass would weigh less. Weight is measured with a scale, like the spring scale in Figure 13.16. The scale measures the force with which gravity pulls an object downward. | text | null |
L_0725 | gravity | T_3588 | People have known about gravity for thousands of years. After all, they constantly experienced gravity in their daily lives. They knew that things always fall toward the ground. However, it wasnt until Sir Isaac Newton developed his law of gravity in the late 1600s that people really began to understand gravity. Newton is pictured in Figure 13.17. | text | null |
L_0725 | gravity | T_3589 | Newton was the first one to suggest that gravity is universal and affects all objects in the universe. Thats why his law of gravity is called the law of universal gravitation. Universal gravitation means that the force that causes an apple to fall from a tree to the ground is the same force that causes the moon to keep moving around Earth. Universal gravitation also means that while Earth exerts a pull on you, you exert a pull on Earth. In fact, there is gravity between you and every mass around you your desk, your book, your pen. Even tiny molecules of gas are attracted to one another by the force of gravity. Newtons law had a huge impact on how people thought about the universe. It explains the motion of objects not only on Earth but in outer space as well. You can learn more about Newtons law of gravity in the video at this URL: | text | null |
L_0725 | gravity | T_3590 | Newtons law also states that the strength of gravity between any two objects depends on two factors: the masses of the objects and the distance between them. Objects with greater mass have a stronger force of gravity. For example, because Earth is so massive, it attracts you and your desk more strongly than you and your desk attract each other. Thats why you and the desk remain in place on the floor rather than moving toward one another. Objects that are closer together have a stronger force of gravity. For example, the moon is closer to Earth than it is to the more massive sun, so the force of gravity is greater between the moon and Earth than between the moon and the sun. Thats why the moon circles around Earth rather than the sun. This is illustrated in Figure You can apply these relationships among mass, distance, and gravity by designing your own roller coaster at this URL: . | text | null |
L_0725 | gravity | T_3591 | Newtons idea of gravity can predict the motion of most but not all objects. In the early 1900s, Albert Einstein came up with a theory of gravity that is better at predicting how all objects move. Einstein showed mathematically that gravity is not really a force in the sense that Newton thought. Instead, gravity is a result of the warping, or curving, of space and time. Imagine a bowling ball pressing down on a trampoline. The surface of the trampoline would curve downward instead of being flat. Einstein theorized that Earth and other very massive bodies affect space and time around them in a similar way. This idea is represented in Figure 13.19. According to Einstein, objects curve toward one another because of the curves in space and time, not because they are pulling on each other with a force of attraction as Newton thought. You can see an animation of Einsteins theory of gravity at this URL: http://einstein. theory of gravity, go to this URL: | text | null |
L_0725 | gravity | T_3591 | Newtons idea of gravity can predict the motion of most but not all objects. In the early 1900s, Albert Einstein came up with a theory of gravity that is better at predicting how all objects move. Einstein showed mathematically that gravity is not really a force in the sense that Newton thought. Instead, gravity is a result of the warping, or curving, of space and time. Imagine a bowling ball pressing down on a trampoline. The surface of the trampoline would curve downward instead of being flat. Einstein theorized that Earth and other very massive bodies affect space and time around them in a similar way. This idea is represented in Figure 13.19. According to Einstein, objects curve toward one another because of the curves in space and time, not because they are pulling on each other with a force of attraction as Newton thought. You can see an animation of Einsteins theory of gravity at this URL: http://einstein. theory of gravity, go to this URL: | text | null |
L_0725 | gravity | T_3592 | Regardless of what gravity is a force between masses or the result of curves in space and time the effects of gravity on motion are well known. You already know that gravity causes objects to fall down to the ground. Gravity affects the motion of objects in other ways as well. | text | null |
L_0725 | gravity | T_3593 | When gravity pulls objects toward the ground, it causes them to accelerate. Acceleration due to gravity equals 9.8 m/s2 . In other words, the velocity at which an object falls toward Earth increases each second by 9.8 m/s. Therefore, after 1 second, an object is falling at a velocity of 9.8 m/s. After 2 seconds, it is falling at a velocity of 19.6 m/s (9.8 m/s 2), and so on. This is illustrated in Figure 13.20. You can compare the acceleration due to gravity on Earth, the moon, and Mars with the interactive animation called "Freefall" at this URL: http://jersey.uoregon.edu/vlab/ . You might think that an object with greater mass would accelerate faster than an object with less mass. After all, its greater mass means that it is pulled by a stronger force of gravity. However, a more massive object accelerates at the same rate as a less massive object. The reason? The more massive object is harder to move because of its greater mass. As a result, it ends up moving at the same acceleration as the less massive object. Consider a bowling ball and a basketball. The bowling ball has greater mass than the basketball. However, if you were to drop both balls at the same time from the same distance above the ground, they would reach the ground together. This is true of all falling objects, unless air resistance affects one object more than another. For example, a falling leaf is slowed down by air resistance more than a falling acorn because of the leafs greater surface area. However, if the leaf and acorn were to fall in the absence of air (that is, in a vacuum), they would reach the ground at the same time. | text | null |
L_0725 | gravity | T_3594 | Earths gravity also affects the acceleration of objects that start out moving horizontally, or parallel to the ground. Look at Figure 13.21. A cannon shoots a cannon ball straight ahead, giving the ball horizontal motion. At the same time, gravity pulls the ball down toward the ground. Both forces acting together cause the ball to move in a curved path. This is called projectile motion. Projectile motion also applies to other moving objects, such as arrows shot from a bow. To hit the bulls eye of a target with an arrow, you actually have to aim for a spot above the bulls eye. Thats because by the time the arrow reaches the target, it has started to curve downward toward the ground. Figure 13.22 shows what happens if you aim at the bulls eye instead of above it. You can access interactive animations of projectile motion at these URLs: http://phet.colorado.edu/en/simulation/projectile-motion http://jersey.uoregon.edu/vlab/ (Select the applet entitled Cannon.) | text | null |
L_0725 | gravity | T_3595 | The moon moves around Earth in a circular path called an orbit. Why doesnt Earths gravity pull the moon down to the ground instead? The moon has enough forward velocity to partly counter the force of Earths gravity. It constantly falls toward Earth, but it stays far enough away from Earth so that it actually falls around the planet. As a result, the moon keeps orbiting Earth and never crashes into it. The diagram in Figure 13.23 shows how this happens. You can explore gravity and orbital motion in depth with the animation at this URL: http://phet.colorado You can see an animated version of this diagram at: http://en.wikipedia.org/wiki/File:Orbital_motion.gif . | text | null |
L_0725 | gravity | T_3595 | The moon moves around Earth in a circular path called an orbit. Why doesnt Earths gravity pull the moon down to the ground instead? The moon has enough forward velocity to partly counter the force of Earths gravity. It constantly falls toward Earth, but it stays far enough away from Earth so that it actually falls around the planet. As a result, the moon keeps orbiting Earth and never crashes into it. The diagram in Figure 13.23 shows how this happens. You can explore gravity and orbital motion in depth with the animation at this URL: http://phet.colorado You can see an animated version of this diagram at: http://en.wikipedia.org/wiki/File:Orbital_motion.gif . | text | null |
L_0726 | elastic force | T_3596 | Something that is elastic can return to its original shape after being stretched or compressed. This property is called elasticity. As you stretch or compress an elastic material, it resists the change in shape. It exerts a counter force in the opposite direction. This force is called elastic force. Elastic force causes the material to spring back to its original shape as soon as the stretching or compressing force is released. You can watch a demonstration of elastic force at this URL: (3:57). MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0726 | elastic force | T_3597 | Elastic force can be very useful. You probably use it yourself every day. A few common uses of elastic force are pictured in Figure 13.25. Did you ever use a resistance band like the one in the figure? When you pull on the band, it stretches but doesnt break. The resistance you feel when you pull on it is elastic force. The resistance of the band to stretching is what gives your muscles a workout. After you stop pulling on the band, it returns to its original shape, ready for the next workout. Springs like the ones in Figure 13.26 also have elastic force when they are stretched or compressed. And like stretchy materials, they return to their original shape when the stretching or compressing force is released. Because of these properties, springs are used in scales to measure weight. They also cushion the ride in a car and provide springy support beneath a mattress. Can you think of other uses of springs? | text | null |
L_0727 | newtons first law | T_3598 | Newtons first law of motion states that an objects motion will not change unless an unbalanced force acts on the object. If the object is at rest, it will stay at rest. If the object is in motion, it will stay in motion and its velocity will remain the same. In other words, neither the direction nor the speed of the object will change as long as the net force acting on it is zero. You can watch a video about Newtons first law at this URL: http://videos.howstuffworks.com/ Look at the pool balls in Figure 14.2. When a pool player pushes the pool stick against the white ball, the white ball is set into motion. Once the white ball is rolling, it rolls all the way across the table and stops moving only after it crashes into the cluster of colored balls. Then, the force of the collision starts the colored balls moving. Some may roll until they bounce off the raised sides of the table. Some may fall down into the holes at the edges of the table. None of these motions will occur, however, unless that initial push of the pool stick is applied. As long as the net force on the balls is zero, they will remain at rest. | text | null |
L_0727 | newtons first law | T_3599 | Newtons first law of motion is also called the law of inertia. Inertia is the tendency of an object to resist a change in its motion. If an object is already at rest, inertia will keep it at rest. If the object is already moving, inertia will keep it moving. Think about what happens when you are riding in a car that stops suddenly. Your body moves forward on the seat. Why? The brakes stop the car but not your body, so your body keeps moving forward because of inertia. Thats why its important to always wear a seat belt. Inertia also explains the amusement park ride in Figure 14.1. The car keeps changing direction, but the riders keep moving in the same direction as before. They slide to the opposite side of the car as a result. You can see an animation of inertia at this URL: | text | null |
L_0727 | newtons first law | T_3600 | The inertia of an object depends on its mass. Objects with greater mass also have greater inertia. Think how hard it would be to push a big box full of books, like the one in Figure 14.3. Then think how easy it would be to push the box if it was empty. The full box is harder to move because it has greater mass and therefore greater inertia. | text | null |
L_0727 | newtons first law | T_3601 | To change the motion of an object, inertia must be overcome by an unbalanced force acting on the object. Until the soccer player kicks the ball in Figure 14.4, the ball remains motionless on the ground. However, when the ball is kicked, the force on it is suddenly unbalanced. The ball starts moving across the field because its inertia has been overcome. | text | null |
L_0728 | newtons second law | T_3602 | Newton determined that two factors affect the acceleration of an object: the net force acting on the object and the objects mass. The relationships between these two factors and motion make up Newtons second law of motion. This law states that the acceleration of an object equals the net force acting on the object divided by the objects mass. This can be represented by the equation: Net force , or Mass F a= m Acceleration = You can watch a video about how Newtons second law of motion applies to football at this URL: http://science36 | text | null |
L_0728 | newtons second law | T_3603 | Newtons second law shows that there is a direct relationship between force and acceleration. The greater the force that is applied to an object of a given mass, the more the object will accelerate. For example, doubling the force on the object doubles its acceleration. The relationship between mass and acceleration, on the other hand, is an inverse relationship. The greater the mass of an object, the less it will accelerate when a given force is applied. For example, doubling the mass of an object results in only half as much acceleration for the same amount of force. Consider the example of a batter, like the boy in Figure 14.6. The harder he hits the ball, the greater will be its acceleration. It will travel faster and farther if he hits it with more force. What if the batter hits a baseball and a softball with the same amount of force? The softball will accelerate less than the baseball because the softball has greater mass. As a result, it wont travel as fast or as far as the baseball. | text | null |
L_0728 | newtons second law | T_3604 | The equation for acceleration given above can be used to calculate the acceleration of an object that is acted on by an unbalanced force. For example, assume you are pushing a large wooden trunk, like the one shown in Figure acceleration of the trunk, substitute these values in the equation for acceleration: a= F 20 N 2N = = m 10 kg kg Recall that one newton (1 N) is the force needed to cause a 1-kilogram mass to accelerate at 1 m/s2 . Therefore, force can also be expressed in the unit kgm/s2 . This way of expressing force can be substituted for newtons in the solution to the problem: a= 2 N 2 kg m/s2 = = 2 m/s2 kg kg Why are there no kilograms in the final answer to this problem? The kilogram units in the numerator and denominator of the fraction cancel out. As a result, the answer is expressed in the correct units for acceleration: m/s2 . You Try It! Problem: Assume that you add the weights to the trunk in Figure 14.7. If you push the trunk and weights with a force of 20 N, what will be the trunks acceleration? Need more practice? You can find additional problems at this URL: | text | null |
L_0728 | newtons second law | T_3605 | Newtons second law of motion explains the weight of objects. Weight is a measure of the force of gravity pulling on an object of a given mass. Its the force (F) in the acceleration equation that was introduced above: a= F m This equation can also be written as: F = ma The acceleration due to gravity of an object equals 9.8 m/s2 , so if you know the mass of an object, you can calculate its weight as: F = m 9.8 m/s2 As this equation shows, weight is directly related to mass. As an objects mass increases, so does its weight. For example, if mass doubles, weight doubles as well. You can learn more about weight and acceleration at this URL: Problem Solving Problem: Daisy has a mass of 35 kilograms. How much does she weigh? Solution: Use the formula: F = m 9.8 m/s2 . F = 35 kg 9.8 m/s2 = 343.0 kg m/s2 = 343.0 N You Try It! Problem: Daisys dad has a mass is 70 kg, which is twice Daisys mass. Predict how much Daisys dad weighs. Then calculate his weight to see if your prediction is correct. Helpful Hints The equation for calculating weight (F = m a) works only when the correct units of measurement are used. Mass must be in kilograms (kg). Acceleration must be in m/s2 . Weight (F) is expressed in kgm/s2 or in newtons (N). | text | null |
L_0729 | newtons third law | T_3606 | Newtons third law of motion states that every action has an equal and opposite reaction. This means that forces always act in pairs. First an action occurs, such as the skateboarders pushing together. Then a reaction occurs that is equal in strength to the action but in the opposite direction. In the case of the skateboarders, they move apart, and the distance they move depends on how hard they first pushed together. You can see other examples of actions and reactions in Figure 14.9. You can watch a video about actions and reactions at this URL: You might think that actions and reactions would cancel each other out like balanced forces do. Balanced forces, which are also equal and opposite, cancel each other out because they act on the same object. Action and reaction forces, in contrast, act on different objects, so they dont cancel each other out and, in fact, often result in motion. For example, in Figure 14.9, the kangaroos action acts on the ground, but the grounds reaction acts on the kangaroo. As a result, the kangaroo jumps away from the ground. One of the action-reaction examples in the Figure 14.9 does not result in motion. Do you know which one it is? | text | null |
L_0729 | newtons third law | T_3607 | What if a friend asked you to play catch with a bowling ball, like the one pictured in Figure 14.10? Hopefully, you would refuse to play! A bowling ball would be too heavy to catch without risk of injury assuming you could even throw it. Thats because a bowling ball has a lot of mass. This gives it a great deal of momentum. Momentum is a property of a moving object that makes the object hard to stop. It equals the objects mass times its velocity. It can be represented by the equation: Momentum = Mass Velocity This equation shows that momentum is directly related to both mass and velocity. An object has greater momentum if it has greater mass, greater velocity, or both. For example, a bowling ball has greater momentum than a softball when both are moving at the same velocity because the bowling ball has greater mass. However, a softball moving at a very high velocity say, 100 miles an hour would have greater momentum than a slow-rolling bowling ball. If an object isnt moving at all, it has no momentum. Thats because its velocity is zero, and zero times anything is zero. | text | null |
L_0729 | newtons third law | T_3608 | Momentum can be calculated by multiplying an objects mass in kilograms (kg) by its velocity in meters per second (m/s). For example, assume that a golf ball has a mass of 0.05 kg. If the ball is traveling at a velocity of 50 m/s, its momentum is: Momentum = 0.05 kg 50 m/s = 2.5 kg m/s Note that the SI unit for momentum is kgm/s. Problem Solving Problem: What is the momentum of a 40-kg child who is running straight ahead with a velocity of 2 m/s? Solution: The child has momentum of: 40 kg 2 m/s = 80 kgm/s. You Try It! Problem: Which football player has greater momentum? Player A: mass = 60 kg; velocity = 2.5 m/s Player B: mass = 65 kg; velocity = 2.0 m/s | text | null |
L_0729 | newtons third law | T_3609 | When an action and reaction occur, momentum is transferred from one object to the other. However, the com- bined momentum of the objects remains the same. In other words, momentum is conserved. This is the law of conservation of momentum. Consider the example of a truck colliding with a car, which is illustrated in Figure 14.11. Both vehicles are moving in the same direction before and after the collision, but the truck is moving faster than the car before the collision occurs. During the collision, the truck transfers some of its momentum to the car. After the collision, the truck is moving slower and the car is moving faster than before the collision occurred. Nonetheless, their combined momentum is the same both before and after the collision. You can see an animation showing how momentum is conserved in a head-on collision at this URL: . | text | null |
L_0729 | newtons third law | T_3610 | Paul Doherty of the Exploratorium performs a "sit-down" lecture on one of Sir Issac Newtons most famous laws. For more information on Newtons laws of motion, see http://science.kqed.org/quest/video/quest-lab-newtons-laws- MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0729 | newtons third law | T_3611 | At UC Berkeley, a team of undergrads is experimenting with velocity, force, and aerodynamics. But you wont find them in a lab they work on a baseball diamond, throwing fast balls, sliders and curve balls. QUEST discovers how the principles of physics can make the difference between a strike and a home run. For more information on the physics of baseball, see http://science.kqed.org/quest/video/out-of-the-park-the-physics-of-baseball/ . MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0731 | buoyancy of fluids | T_3623 | Buoyancy is the ability of a fluid to exert an upward force on any object placed in the fluid. This upward force is called buoyant force. | text | null |
L_0731 | buoyancy of fluids | T_3624 | What explains buoyant force? Recall from the earlier lesson "Pressure of Fluids" that a fluid exerts pressure in all directions but the pressure is greater at greater depth. Therefore, the fluid below an object exerts greater force on the object than the fluid above the object. This is illustrated in Figure 15.12. Buoyant force explains why objects may float in water. No doubt youve noticed, however, that some objects do not float in water. If buoyant force applies to all objects in fluids, why do some objects sink instead of float? The answer has to do with their weight. | text | null |
L_0731 | buoyancy of fluids | T_3625 | Weight is a measure of the force of gravity pulling down on an object. Buoyant force pushes up on an object. Weight and buoyant force together determine whether an object sinks or floats. This is illustrated in Figure 15.13. If an objects weight is the same as the buoyant force acting on the object, then the object floats. This is the example on the left in Figure 15.13. If an objects weight is greater than the buoyant force acting on the object, then the object sinks. This is the example on the right in Figure 15.13. Because of buoyant force, objects seem lighter in water. You may have noticed this when you went swimming and could easily pick up a friend or sibling under the water. Some of the persons weight was countered by the buoyant force of the water. | text | null |
L_0731 | buoyancy of fluids | T_3626 | Density, or the amount of mass in a given volume, is also related to buoyancy. Thats because density affects weight. A given volume of a denser substance is heavier than the same volume of a less dense substance. For example, ice is less dense than liquid water. This explains why ice cubes float in a glass of water. This and other examples of density and buoyant force are illustrated in Figure 15.14 and in the video at this URL: MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0731 | buoyancy of fluids | T_3627 | Did you ever notice that when you get into a bathtub of water the level of the water rises? More than 2200 years ago, a Greek mathematician named Archimedes noticed the same thing. He observed that both a body and the water in a tub cant occupy the same space at the same time. As a result, some of the water is displaced, or moved out of the way. How much water is displaced? Archimedes determined that the volume of displaced water equals the volume of the submerged object. So more water is displaced by a bigger body than a smaller one. What does displacement have to do with buoyant force? Everything! Archimedes discovered that the buoyant force acting on an object in a fluid equals the weight of the fluid displaced by the object. This is known as Archimedes law (or Archimedes Principle). Archimedes law explains why some objects float in fluids even though they are very heavy. Remember the oil tanker that opened this chapter? It is extremely heavy, yet it stays afloat. If a steel ball with the same weight as the ship were put into water, it would sink to the bottom (see Figure 15.15). Thats because the volume of water displaced by the steel ball weighs less than the ball. As a result, the buoyant force is not as great as the force of gravity acting on the ball. The design of the ships hull, on the other hand, causes it to displace much more water than the ball. In fact, the weight of the displaced water is greater than the weight of the ship, so the buoyant force is greater than the force of gravity acting on the ship. As a result, the ship floats. You can check your understanding of Archimedes law by doing the brainteaser at this URL: . For an entertaining video presentation of Archimedes law, go to this URL: http://videos.howstuffworks.com/disc | text | null |
L_0732 | work | T_3628 | Work is defined differently in physics than in everyday language. In physics, work means the use of force to move an object. The teen who is playing tennis in Figure 16.1 is using force to move her tennis racket, so she is doing work. The teen who is studying isnt moving anything, so she is not doing work. Not all force that is used to move an object does work. For work to be done, the force must be applied in the same direction that the object moves. If a force is applied in a different direction than the object moves, no work is done. Figure 16.2 illustrates this point. The stick person applies an upward force on the box when raising it from the ground to chest height. Work is done because the force is applied in the same direction as the box is moving. However, as the stick person walks from left to right while holding the box at chest height, no more work is done by the persons arms holding the box up. Thats because the force supporting the box acts in a different direction than the box is moving. A small amount of work in the horizontal direction is performed when the person is accelerating during the first step of the walk across the room. But other than that, there is no work, because there is no net force acting on the box horizontally. | text | null |
L_0732 | work | T_3629 | Work is directly related to both the force applied to an object and the distance the object moves. It can be represented by the equation: Work = Force Distance This equation shows that the greater the force that is used to move an object or the farther the object is moved, the more work that is done. You can see a short video introduction to work as the product of force and distance at this link: . To see the effects of force and distance on work, compare the weight lifters in Figure 16.3. The two weight lifters on the left are lifting the same amount of weight, but the bottom weight lifter is lifting the weight a longer distance. Therefore, this weight lifter is doing more work. The two weight lifters on the bottom right are both lifting the weight the same distance, but the weight lifter on the left is lifting a heavier weight. Therefore, this weight lifter is doing more work. | text | null |
L_0732 | work | T_3630 | The equation for work given above can be used to calculate the amount of work that is done if force and distance are known. For example, assume that one of the weight lifters in Figure 16.2 lifts a weight of 400 newtons over his head to a height of 2.2 meters off the ground. The amount of work he does is: Work = 400 N 2.2 m = 880 N m Notice that the unit for work is the newton meter. This is the SI unit for work, also called the joule (J). One joule equals the amount of work that is done when 1 newton of force moves an object over a distance of 1 meter. Problem Solving Problem: Todd pushed a 500 N box 4 meters across the floor. How much work did he do? Solution: Use the equation Work = Force Distance. Work = 500 N 4 m = 2000 N m, or 2000 J You Try It! Problem: Lara lifted a 100 N box 1.5 meters above the floor. How much work did she do? | text | null |
L_0732 | work | T_3631 | Did you ever rake leaves, like the woman in Figure 16.4? It can take a long time to do all that work. But if you use an electric leaf blower, like the man in the figure, the job gets done much sooner. Both the leaf blower and the rake do the work of removing leaves from the yard, but the leaf blower has more power. Thats why it can do the same amount of work in less time. | text | null |
L_0732 | work | T_3632 | Power is a measure of the amount of work that can be done in a given amount of time. Power can be represented by the equation: Power = Work Time In this equation, work is measured in joules and time is measured in seconds, so power is expressed in joules per second (J/s). This is the SI unit for work, also known as the watt (W). A watt equals 1 joule of work per second. The watt is named for James Watt, a Scottish inventor you will read about below. You may already be familiar with watts. Thats because light bulbs and small appliances such as hair dryers are labeled with the watts of power they provide. For example, the hair dryer in Figure 16.5 is labeled "2000 watts." This amount of power could also be expressed kilowatts. A kilowatt equals 1000 watts, so the 2000-watt hair dryer produces 2 kilowatts of power. Compared with a less powerful device, a more powerful device can either do more work in the same time or do the same work in less time. For example, compared with a low-power microwave, a high-power microwave can cook more food in the same time or the same amount of food in less time. | text | null |
L_0732 | work | T_3632 | Power is a measure of the amount of work that can be done in a given amount of time. Power can be represented by the equation: Power = Work Time In this equation, work is measured in joules and time is measured in seconds, so power is expressed in joules per second (J/s). This is the SI unit for work, also known as the watt (W). A watt equals 1 joule of work per second. The watt is named for James Watt, a Scottish inventor you will read about below. You may already be familiar with watts. Thats because light bulbs and small appliances such as hair dryers are labeled with the watts of power they provide. For example, the hair dryer in Figure 16.5 is labeled "2000 watts." This amount of power could also be expressed kilowatts. A kilowatt equals 1000 watts, so the 2000-watt hair dryer produces 2 kilowatts of power. Compared with a less powerful device, a more powerful device can either do more work in the same time or do the same work in less time. For example, compared with a low-power microwave, a high-power microwave can cook more food in the same time or the same amount of food in less time. | text | null |
L_0732 | work | T_3633 | Power can be calculated using the formula above, if the amount of work and time are known. For example, assume that a small engine does 3000 joules of work in 2 seconds. Then the power of the motor is: Power = 3000 J = 1500 J/s, or 1500 W 2s You can also calculate work if you know power and time by rewriting the power equation above as: Work = Power Time For example, if you use a 2000-watt hair dryer for 30 seconds, how much work is done? First express 2000 watts in J/s and then substitute this value for power in the work equation: Work = 2000 J/s 30 s = 60, 000 J For a video presentation on calculating power and work, go to this link: Problem Solving Problem: An electric mixer does 2500 joules of work in 5 seconds. What is its power? Solution: Use the equation: Power = Work Time . Power = 2500 J = 500 J/s, or 500 W 5s You Try It! Problem: How much work can be done in 30 seconds by a 1000-watt microwave? | text | null |
L_0732 | work | T_3634 | Sometimes power is measured in a unit called the horsepower. One horsepower is the amount of work a horse can do in 1 minute. It equals 745 watts of power. The horsepower was introduced by James Watt, who invented the first powerful steam engine in the 1770s. Watts steam engine led to a revolution in industry and agriculture because of its power. Watt wanted to impress people with the power of his steam engine, so he compared it with something familiar to people of his time: the power of workhorses, like those pictured in Figure 16.6. Watt said his steam engine could produce the power of 20 horses, or 20 horsepower. The most powerful engines today may produce more than 100,000 horsepower! How many watts of power is that? | text | null |
L_0733 | machines | T_3635 | A machine is any device that makes work easier by changing a force. When you use a machine, you apply force to the machine. This force is called the input force. The machine, in turn, applies force to an object. This force is called the output force. Recall that work equals force multiplied by distance: Work = Force Distance The force you apply to a machine is applied over a given distance, called the input distance. The force applied by the machine to the object is also applied over a distance, called the output distance. The output distance may or may not be the same as the input distance. Machines make work easier by increasing the amount of force that is applied, increasing the distance over which the force is applied, or changing the direction in which the force is applied. Contrary to popular belief, machines do not increase the amount of work that is done. They just change how the work is done. So if a machine increases the force applied, it must apply the force over a shorter distance. Similarly, if a machine increases the distance over which the force is applied, it must apply less force. | text | null |
L_0733 | machines | T_3636 | Examples of machines that increase force are doorknobs and nutcrackers. Figure 16.8 explains how these machines work. In each case, the force applied by the user is less than the force applied by the machine, but the machine applies the force over a shorter distance. | text | null |
L_0733 | machines | T_3637 | Examples of machines that increase the distance over which force is applied are paddles and hammers. Figure 16.9 explains how these machines work. In each case, the machine increases the distance over which the force is applied, but it reduces the strength of the applied force. | text | null |
L_0733 | machines | T_3638 | Some machines change the direction of the force applied by the user. They may or may not also change the strength of the force or the distance over which it is applied. Two examples of machines that work in this way are claw hammers and the rope systems (pulleys) that raise or lower flags on flagpoles. Figure 16.10 explains how these machines work. In each case, the direction of the force applied by the user is reversed by the machine. How does this make it easier to do the job? | text | null |
L_0733 | machines | T_3639 | An exoskeleton suit may seem like science fiction, turning ordinary humans into super heroes. But wearable robots are moving forward into reality. And for paraplegics, the ability to stand and walk that these machines provide is a super power. QUEST meets Austin Whitney and Tamara Mena, two "Exoskeleton Test Pilots" who are now putting this new technology through its paces. For more information on exoskeleton suits, see http://science.kqed.org/ques MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0733 | machines | T_3640 | You read above that machines do not increase the work done on an object. In other words, you cant get more work out of a machine than you put into it. In fact, machines always do less work on the object than the user does on the machine. Thats because all machines must use some of the work put into them to overcome friction. How much work? It depends on the efficiency of the machine. Efficiency is the percent of input work that becomes output work. It is a measure of how well a machine reduces friction. | text | null |
L_0733 | machines | T_3641 | Consider the ramp in Figure 16.11. Its easier to push the heavy piece of furniture up the ramp to the truck than to lift it straight up off the ground. However, pushing the furniture over the surface of the ramp creates a lot of friction. Some of the force applied to moving the furniture must be used to overcome the friction. It would be more efficient to use a dolly on wheels to roll the furniture up the ramp. Thats because rolling friction is much less than sliding friction. As a result, the efficiency of the ramp would be greater with a dolly. | text | null |
L_0733 | machines | T_3642 | Efficiency can be calculated with the equation: Efficiency = Output work 100% Input work Consider a machine that puts out 6000 joules of work. To produce that much work from the machine requires the user to put in 8000 joules of work. To find the efficiency of the machine, substitute these values into the equation for efficiency: Efficiency = 6000 J 100% = 75% 8000 J You Try It! Problem: Rani puts 10,000 joules of work into a car jack. The car jack, in turn, puts out 7000 joules of work to raise up the car. What is the efficiency of the jack? | text | null |
L_0733 | machines | T_3643 | Another measure of the effectiveness of a machine is its mechanical advantage. Mechanical advantage is the number of times a machine multiplies the input force. It can be calculated with the equation: Mechanical Advantage = Output force Input force This equation computes the actual mechanical advantage of a machine. It takes into account the reduction in output force that is due to friction. It shows how much a machine actually multiplies force when it used in the real world. | text | null |
L_0733 | machines | T_3644 | It can be difficult to measure the input and output forces needed to calculate actual mechanical advantage. Its usually much easier to measure the input and output distances. These measurements can then be used to calculate the ideal mechanical advantage. The ideal mechanical advantage represents the multiplication of input force that would be achieved in the absence of friction. Therefore, it is greater than the actual mechanical advantage because all machines use up some work in overcoming friction. Ideal mechanical advantage is calculated with the equation: Ideal Mechanical Advantage = Input distance Output distance Compare this equation with the equation above for actual mechanical advantage. Notice how the input and output values are switched. This makes sense when you recall that when a machine increases force, it decreases distance and vice versa. You can watch a video about actual and ideal mechanical advantage at this link: http://video.goo Consider the simple ramp in Figure 16.12. A ramp can be used to raise an object up off the ground. The input distance is the length of the sloped surface of the ramp. The output distance is the height of the ramp, or the vertical distance the object is raised. Therefore, the ideal mechanical advantage of the ramp is: Ideal Mechanical Advantage = 6m =3 2m An ideal mechanical advantage of 3 means that the ramp ideally (in the absence of friction) multiplies the output force by a factor of 3. | text | null |
L_0733 | machines | T_3645 | As you read above, some machines increase the force put into the machine, while other machines increase the distance over which the force is applied. Still other machines change only the direction of the force. Which way a machine works affects its mechanical advantage. For machines that increase force including ramps, doorknobs, and nutcrackers the output force is greater than the input force. Therefore, the mechanical advantage is greater than 1. For machines that increase the distance over which force is applied, such as paddles and hammers, the output force is less than the input force. Therefore, the mechanical advantage is less than 1. For machines that change only the direction of the force, such as the rope systems on flagpoles, the output force is the same as the input force. Therefore, the mechanical advantage is equal to 1. | text | null |
L_0734 | simple machines | T_3646 | The man in Figure 16.14 is using a ramp to move a heavy dryer up to the back of a truck. The highway in the figure switches back and forth so it climbs up the steep hillside. Both the ramp and the highway are examples of inclined planes. An inclined plane is a simple machine consisting of a sloping surface that connects lower and higher elevations. The sloping surface of the inclined plane supports part of the weight of the object as it moves up the slope. As a result, it takes less force to move the object uphill. The trade-off is that the object must be moved over a greater distance than if it were moved straight up to the higher elevation. On the other hand, the output force is greater than the input force because it is applied over a shorter distance. Like other simple machines, the ideal mechanical advantage of an inclined plane is given by: Ideal Mechanical Advantage = Input distance Output distance For an inclined plane, the input distance is the length of the sloping surface, and the output distance is the maximum height of the inclined plane. This was illustrated in Figure 16.12. Because the sloping surface is always greater than the height of the inclined plane, the ideal mechanical advantage of an inclined plane is always greater than 1. An inclined plane with a longer sloping surface relative to its height has a gentler slope. An inclined plane with a gentler slope has a greater mechanical advantage and requires less input force to move an object to a higher elevation. | text | null |
L_0734 | simple machines | T_3647 | Two simple machines that are based on the inclined plane are the wedge and the screw. Both increase the force used to move an object because the input force is applied over a greater distance than the output force. | text | null |
L_0734 | simple machines | T_3648 | Imagine trying to slice a tomato with a fork or spoon instead of a knife, like the one in Figure 16.15. The knife makes the job a lot easier because of the wedge shape of the blade. A wedge is a simple machine that consists of two inclined planes. But unlike one inclined plane, a wedge works only when it moves. It has a thin end and thick end, and the thin end is forced into an object to cut or split it. The chisel in Figure 16.15 is another example of a wedge. The input force is applied to the thick end of a wedge, and it acts over the length of the wedge. The output force pushes against the object on both sides of the wedge, so the output distance is the thickness of the wedge. Therefore, the ideal mechanical advantage of a wedge can be calculated as: Ideal Mechanical Advantage = Length of wedge Maximum thickness of wedge The length of a wedge is always greater than its maximum thickness. As a result, the ideal mechanical advantage of a wedge is always greater than 1. | text | null |
L_0734 | simple machines | T_3649 | The spiral staircase in Figure 16.16 also contains an inclined plane. Do you see it? The stairs that wrap around the inside of the walls make up the inclined plane. The spiral staircase is an example of a screw. A screw is a simple machine that consists of an inclined plane wrapped around a cylinder or cone. No doubt you are familiar with screws like the wood screw in Figure 16.16. The screw top of the container in the figure is another example. Screws move objects to a higher elevation (or greater depth) by increasing the force applied. When you use a wood screw, you apply force to turn the inclined plane. The output force pushes the screw into the wood. It acts along the length of the cylinder around which the inclined plane is wrapped. Therefore, the ideal mechanical advantage of a screw is calculated as: Ideal Mechanical Advantage = Length of inclined plane Length of screw The length of the inclined plane is always greater than the length of the screw. As a result, the mechanical advantage of a screw is always greater than 1. Look at the collection of screws and bolts in Figure 16.17. In some of them, the turns (or threads) of the inclined plane are closer together. The closer together the threads are, the longer the inclined plane is relative to the length of the screw or bolt, so the greater its mechanical advantage is. Therefore, if the threads are closer together, you need to apply less force to penetrate the wood or other object. The trade-off is that more turns of the screw or bolt are needed to do the job because the distance over which the input force must be applied is greater. | text | null |
L_0734 | simple machines | T_3649 | The spiral staircase in Figure 16.16 also contains an inclined plane. Do you see it? The stairs that wrap around the inside of the walls make up the inclined plane. The spiral staircase is an example of a screw. A screw is a simple machine that consists of an inclined plane wrapped around a cylinder or cone. No doubt you are familiar with screws like the wood screw in Figure 16.16. The screw top of the container in the figure is another example. Screws move objects to a higher elevation (or greater depth) by increasing the force applied. When you use a wood screw, you apply force to turn the inclined plane. The output force pushes the screw into the wood. It acts along the length of the cylinder around which the inclined plane is wrapped. Therefore, the ideal mechanical advantage of a screw is calculated as: Ideal Mechanical Advantage = Length of inclined plane Length of screw The length of the inclined plane is always greater than the length of the screw. As a result, the mechanical advantage of a screw is always greater than 1. Look at the collection of screws and bolts in Figure 16.17. In some of them, the turns (or threads) of the inclined plane are closer together. The closer together the threads are, the longer the inclined plane is relative to the length of the screw or bolt, so the greater its mechanical advantage is. Therefore, if the threads are closer together, you need to apply less force to penetrate the wood or other object. The trade-off is that more turns of the screw or bolt are needed to do the job because the distance over which the input force must be applied is greater. | text | null |
L_0734 | simple machines | T_3650 | Did you ever use a hammer to pull a nail out of a board? If not, you can see how its done in Figure 16.18. When you pull down on the handle of the hammer, the claw end pulls up on the nail. A hammer is an example of a lever. A lever is a simple machine consisting of a bar that rotates around a fixed point called the fulcrum. For a video introduction to levers using skateboards as examples, go to this link: MEDIA Click image to the left or use the URL below. URL: A lever may or may not increase the force applied, and it may or may not change the direction of the force. It all depends on the location of the input and output forces relative to the fulcrum. In this regard, there are three basic types of levers, called first-class, second-class, and third-class levers. Figure 16.19 describes the three classes. | text | null |
L_0734 | simple machines | T_3651 | All three classes of levers make work easier, but they do so in different ways. When the input and output forces are on opposite sides of the fulcrum, the lever changes the direction of the applied force. This occurs only with a first-class lever. When both the input and output forces are on the same side of the fulcrum, the direction of the applied force does not change. This occurs with both second- and third-class levers. When the input force is applied farther from the fulcrum, the input distance is greater than the output distance, so the ideal mechanical advantage is greater than 1. This always occurs with second-class levers and may occur with first-class levers. When the input force is applied closer to the fulcrum, the input distance is less than the output distance, so the ideal mechanical advantage is less than 1. This always occurs with third-class levers and may occur with first-class levers. When both forces are the same distance from the fulcrum, the input distance equals the output distance, so the ideal mechanical advantage equals 1. This occurs only with first class-levers. | text | null |
L_0734 | simple machines | T_3652 | You may be wondering why you would use a third-class lever when it doesnt change the direction or strength of the applied force. The advantage of a third-class lever is that the output force is applied over a greater distance than the input force. This means that the output end of the lever must move faster than the input end. Why would this be useful when you are moving a hockey stick or baseball bat, both of which are third-class levers? | text | null |
L_0734 | simple machines | T_3653 | Did you ever ride on a Ferris wheel, like the one pictured in Figure 16.20? If you did, then you know how thrilling the ride can be. A Ferris wheel is an example of a wheel and axle. A wheel and axle is a simple machine that consists of two connected rings or cylinders, one inside the other, which both turn in the same direction around a single center point. The smaller, inner ring or cylinder is called the axle. The bigger, outer ring or cylinder is called the wheel. The car steering wheel in Figure 16.20 is another example of a wheel and axle. In a wheel and axle, force may be applied either to the wheel or to the axle. In both cases, the direction of the force does not change, but the force is either increased or applied over a greater distance. When the input force is applied to the axle, as it is with a Ferris wheel, the wheel turns with less force, so the ideal mechanical advantage is less than 1. However, the wheel turns over a greater distance, so it turns faster than the axle. The speed of the wheel is one reason that the Ferris wheel ride is so exciting. When the input force is applied to the wheel, as it is with a steering wheel, the axle turns over a shorter distance but with greater force, so the ideal mechanical advantage is greater than 1. This allows you to turn the steering wheel with relatively little effort, while the axle of the steering wheel applies enough force to turn the car. | text | null |
L_0734 | simple machines | T_3654 | Another simple machine that uses a wheel is the pulley. A pulley is a simple machine that consists of a rope and grooved wheel. The rope fits into the groove in the wheel, and pulling on the rope turns the wheel. Figure 16.21 shows two common uses of pulleys. Some pulleys are attached to a beam or other secure surface and remain fixed in place. They are called fixed pulleys. Other pulleys are attached to the object being moved and are moveable themselves. They are called moveable pulleys. Sometimes, fixed and moveable pulleys are used together. They make up a compound pulley. The three types of pulleys are compared in Figure 16.22. In all three types, the ideal mechanical advantage is equal to the number of rope segments pulling up on the object. The more rope segments that are helping to do the lifting work, the less force that is needed for the job. You can experiment with an interactive animation of compound pulleys with various numbers of pulleys at this link: . In a single fixed pulley, only one rope segment lifts the object, so the ideal mechanical advantage is 1. This type of pulley doesnt increase the force, but it does change the direction of the force. This allows you to use your weight to pull on one end of the rope and more easily raise the object attached to the other end. In a single moveable pulley, two rope segments lift the object, so the ideal mechanical advantage is 2. This type of pulley doesnt change the direction of the force, but it does increase the force. In a compound pulley, two or more rope segments lift the object, so the ideal mechanical advantage is equal to or greater than 2. This type of pulley may or may not change the direction of the force, depending on the number and arrangement of pulleys. When several pulleys are combined, the increase in force may be very great. To learn more about the mechanical advantage of different types of pulleys, watch the video at this link: http://video | text | null |
L_0735 | compound machines | T_3655 | A compound machine is a machine that consists of more than one simple machine. Some compound machines consist of just two simple machines. For example, a wheelbarrow consists of a lever, as you read earlier in the lesson "Simple Machines," and also a wheel and axle. Other compound machines, such as cars, consist of hundreds or even thousands of simple machines. Two common examples of compound machines are scissors and fishing rods with reels. To view a young students compound machine invention that includes several simple machines, watch the video at this link: . To see if you can identify the simple machines in a lawn mower, go to the URL below and click on Find the Simple Machines. | text | null |
L_0735 | compound machines | T_3656 | Look at the scissors in Figure 16.24. As you can see from the figure, scissors consist of two levers and two wedges. You apply force to the handle ends of the levers, and the output force is exerted by the blade ends of the levers. The fulcrum of both levers is where they are joined together. Notice that the fulcrum lies between the input and output points, so the levers are first-class levers. They change the direction of force. They may or may not also increase force, depending on the relative lengths of the handles and blades. The blades themselves are wedges, with a sharp cutting edge and a thicker dull edge. | text | null |
L_0735 | compound machines | T_3657 | The fishing rod with reel shown in Figure 16.25 is another compound machine. The rod is a third-class lever, with the fulcrum on one end of the rod, the input force close to the fulcrum, and the output force at the other end of the rod. The output distance is greater than the input distance, so the angler can fling the fishing line far out into the water with just a flick of the wrist. The reel is a wheel and axle that works as a pulley. The fishing line is wrapped around the wheel. Using the handle to turn the axle of the wheel winds or unwinds the line. | text | null |
L_0735 | compound machines | T_3658 | Riding a bicycle might be easy. But the forces that allow humans to balance atop a bicycle are complex. QUEST visits Davis a city that loves its bicycles to take a ride on a research bicycle and explore a collection of antique bicycles. Scientists say studying the complicated physics of bicycling can lead to the design of safer, and more efficient bikes. For more information on the science of riding a bicycle, see MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0735 | compound machines | T_3659 | Because compound machines have more moving parts than simple machines, they generally have more friction to overcome. As a result, compound machines tend to have lower efficiency than simple machines. When a compound machine consists of a large number of simple machines, friction may become a serious problem, and it may produce a lot of heat. Lubricants such as oil or grease may be used to coat the moving parts so they slide over each other more easily. This is how a cars friction is reduced. Compound machines have a greater mechanical advantage than simple machines. Thats because the mechanical advantage of a compound machine equals the product of the mechanical advantages of all its component simple machines. The greater the number of simple machines it contains, the greater is its mechanical advantage. | text | null |
L_0736 | types of energy | T_3660 | The concept of energy was first introduced in the chapter "States of Matter," where it is defined as the ability to cause change in matter. Energy can also be defined as the ability to do work. Work is done whenever a force is used to move matter. When work is done, energy is transferred from one object to another. For example, when the batter in Figure 17.2 uses energy to swing the bat, she transfers energy to the bat. The moving bat, in turn, transfers energy to the ball. Like work, energy is measured in the joule (J), or newtonmeter (Nm). Energy exists in different forms, which you can read about in the lesson "Forms of Energy" later in the chapter. Some forms of energy are mechanical, electrical, and chemical energy. Most forms of energy can also be classified as kinetic or potential energy. Kinetic and potential forms of mechanical energy are the focus of this lesson. Mechanical energy is the energy of objects that are moving or have the potential to move. | text | null |
L_0736 | types of energy | T_3661 | What do all the photos in Figure 17.3 have in common? All of them show things that are moving. Kinetic energy is the energy of moving matter. Anything that is moving has kinetic energy from the atoms in matter to the planets in solar systems. Things with kinetic energy can do work. For example, the hammer in the photo is doing the work of pounding the nail into the board. You can see a cartoon introduction to kinetic energy and its relation to work at this URL: . The amount of kinetic energy in a moving object depends on its mass and velocity. An object with greater mass or greater velocity has more kinetic energy. The kinetic energy of a moving object can be calculated with the equation: 1 Kinetic Energy (KE) = mass velocity2 2 This equation for kinetic energy shows that velocity affects kinetic energy more than mass does. For example, if mass doubles, kinetic energy also doubles. But if velocity doubles, kinetic energy increases by a factor of four. Thats because velocity is squared in the equation. You can see for yourself how mass and velocity affect kinetic energy by working through the problems below. Problem Solving Problem: Juan has a mass of 50 kg. If he is running at a velocity of 2 m/s, how much kinetic energy does he have? Solution: Use the formula: KE = 12 mass velocity2 1 50 kg (2 m/s2 ) 2 = 100 kg m2 /s2 = 100 N m, or 100 J KE = You Try It! Problem: What is Juans kinetic energy if he runs at a velocity of 4 m/s? Problem: Juans dad has a mass of 100 kg. How much kinetic energy does he have if he runs at a velocity of 2 m/s? | text | null |
L_0736 | types of energy | T_3662 | Did you ever see a scene like the one in Figure 17.4? In many parts of the world, trees lose their leaves in autumn. The leaves turn color and then fall from the trees to the ground. As the leaves are falling, they have kinetic energy. While they are still attached to the trees they also have energy, but its not because of motion. Instead, they have stored energy, called potential energy. An object has potential energy because of its position or shape. For example leaves on trees have potential energy because they could fall due to the pull of gravity. | text | null |
L_0736 | types of energy | T_3663 | Potential energy due to the position of an object above Earth is called gravitational potential energy. Like the leaves on trees, anything that is raised up above Earths surface has the potential to fall because of gravity. You can see examples of people with gravitational potential energy in Figure 17.5. Gravitational potential energy depends on an objects weight and its height above the ground. It can be calculated with the equation: Gravitational potential energy (GPE) = weight height Consider the diver in Figure 17.5. If he weighs 70 newtons and the diving board is 5 meters above Earths surface, then his potential energy is: GPE = 70 N 5 m = 350 N m, or 350 J | text | null |
L_0736 | types of energy | T_3664 | Potential energy due to an objects shape is called elastic potential energy. This energy results when elastic objects are stretched or compressed. Their elasticity gives them the potential to return to their original shape. For example, the rubber band in Figure 17.6 has been stretched, but it will spring back to its original shape when released. Springs like the handspring in the figure have elastic potential energy when they are compressed. What will happen when the handspring is released? | text | null |
L_0736 | types of energy | T_3665 | Remember the diver in Figure 17.5? What happens when he jumps off the diving board? His gravitational potential energy changes to kinetic energy as he falls toward the water. However, he can regain his potential energy by getting out of the water and climbing back up to the diving board. This requires an input of kinetic energy. These changes in energy are examples of energy conversion, the process in which energy changes from one type or form to another. | text | null |
L_0736 | types of energy | T_3666 | The law of conservation of energy applies to energy conversions. Energy is not used up when it changes form, although some energy may be used to overcome friction, and this energy is usually given off as heat. For example, the divers kinetic energy at the bottom of his fall is the same as his potential energy when he was on the diving board, except for a small amount of heat resulting from friction with the air as he falls. | text | null |
L_0736 | types of energy | T_3667 | There are many other examples of energy conversions between potential and kinetic energy. Figure 17.7 describes how potential energy changes to kinetic energy and back again on swings and trampolines. You can see an animation of changes between potential and kinetic energy on a ramp at the URL below. Can you think of other examples? | text | null |
L_0736 | types of energy | T_3668 | QUEST teams up with Make Magazine to construct the latest must have, do-it-yourself device hacks and science projects. This week well show you how to make a tabletop linear accelerator that demonstrates the finer points of kinetic energy by shooting a steel ball. For more information on the tabletop linear accelerator, see http://science.k MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0737 | forms of energy | T_3669 | Energy, or the ability to do work, can exist in many different forms. The photo in Figure 17.8 represents six of the eight different forms of energy that are described in this lesson. The guitarist gets the energy he needs to perform from chemical energy in food. He uses mechanical energy to pluck the strings of the guitar. The stage lights use electrical energy and give off both light energy and thermal energy, commonly called heat. The guitar also uses electrical energy, and it produces sound energy when the guitarist plucks the strings. For an introduction to all these forms of energy, go to this URL: . For an interactive animation about the different forms of energy, visit this URL: After you read below about different forms of energy, you can check your knowledge by doing the drag and drop quiz at this URL: . | text | null |
L_0737 | forms of energy | T_3670 | Mechanical energy is the energy of an object that is moving or has the potential to move. It is the sum of an objects kinetic and potential energy. In Figure 17.9, the basketball has mechanical energy because it is moving. The arrow in the same figure has mechanical energy because it has the potential to move due to the elasticity of the bow. What are some other examples of mechanical energy? | text | null |
L_0737 | forms of energy | T_3671 | Energy is stored in the bonds between atoms that make up compounds. This energy is called chemical energy, and it is a form of potential energy. If the bonds between atoms are broken, the energy is released and can do work. The wood in the fireplace in Figure 17.10 has chemical energy. The energy is released as thermal energy when the wood burns. People and many other living things meet their energy needs with chemical energy stored in food. When food molecules are broken down, the energy is released and may be used to do work. | text | null |
L_0737 | forms of energy | T_3672 | Electrons are negatively charged particles in atoms. Moving electrons have a form of kinetic energy called electrical energy. If youve ever experienced an electric outage, then you know how hard it is to get by without electrical energy. Most of the electrical energy we use is produced by power plants and arrives in our homes through wires. Two other sources of electrical energy are pictured in Figure 17.11. | text | null |
L_0737 | forms of energy | T_3673 | The nuclei of atoms are held together by powerful forces. This gives them a tremendous amount of stored energy, called nuclear energy. The energy can be released and used to do work. This happens in nuclear power plants when nuclei fission, or split apart. It also happens in the sun and other stars when nuclei fuse, or join together. Some of the suns energy travels to Earth, where it warms the planet and provides the energy for photosynthesis (see Figure | text | null |
L_0737 | forms of energy | T_3674 | The atoms that make up matter are in constant motion, so they have kinetic energy. All that motion gives matter thermal energy. Thermal energy is defined as the total kinetic energy of all the atoms that make up an object. It depends on how fast the atoms are moving and how many atoms the object has. Therefore, an object with more mass has greater thermal energy than an object with less mass, even if their individual atoms are moving at the same speed. You can see an example of this in Figure 17.13. | text | null |
L_0737 | forms of energy | T_3674 | The atoms that make up matter are in constant motion, so they have kinetic energy. All that motion gives matter thermal energy. Thermal energy is defined as the total kinetic energy of all the atoms that make up an object. It depends on how fast the atoms are moving and how many atoms the object has. Therefore, an object with more mass has greater thermal energy than an object with less mass, even if their individual atoms are moving at the same speed. You can see an example of this in Figure 17.13. | text | null |
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