Physics CP Chapter 6 – Flashcards
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In the simplest sense, a force is a push or a pull. A mutual action is an interaction between one thing and another. When you push on the wall, the wall pushes on you. The interaction that drives the nail is the same as the one that halts the hammer.
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Force
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Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object. Describes the relationship between two forces in an interaction. One force is called the action force. The other force is called the reaction force. Neither force exists without the other. They are equal in strength and opposite in direction. They occur at the same time (simultaneously). Newton's third law is often stated: "To every action there is always an equal opposing reaction." It doesn't matter which force we call action and which we call reaction.
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Newton's 3rd Law
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When the girl jumps to shore, the boat moves backward. A person trying to walk on ice, where friction is minimal, may not be able to exert an action force against the ice. Without the action force there cannot be a reaction force, and thus there is no resulting forward motion. The dog wags the tail and the tail wags the dog.
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Examples
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To identify a pair of action-reaction forces, first identify the interacting objects A and B, and if the action is A on B, the reaction is B on A. When action is A exerts force on B, the reaction is simply B exerts force on A.
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Identifying Action and Reaction
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The cannonball undergoes more acceleration than the cannon because its mass is much smaller. F represents both the action and reaction forces; m (large), the mass of the cannon; and m (small), the mass of the cannonball. The balloon recoils from the escaping air and climbs upward. The rocket recoils from the "molecular cannonballs" it fires and climbs upward.
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Action and Reaction on Different Masses
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Using Newton's third law, we can understand how a helicopter gets its lifting force. The whirling blades force air particles downward (action). The air forces the blades upward (reaction). This upward reaction force is called lift. When lift equals the weight of the craft, the helicopter hovers in midair. When lift is greater, the helicopter climbs upward. Birds and airplanes also fly because of action and reaction forces. When a bird is soaring, the shape of its wings deflects air downward. The air in turn pushes the bird upward. The slightly tilted wings of an airplane also deflect oncoming air downward and produce lift.
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Lift
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They pull on each other with equal, but opposite forces. The reason the Moon is not pulled into Earth is because it's orbiting at a velocity that gives it centrifugal force that offsets the pull of gravity. We think the Moon is revolving around the Earth, but if we were on the Moon, the Earth would appear to revolve around the Moon. So Earth, too, has a centrifugal force keeping it from closing in on the Moon.
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We know that Earth pulls on the moon. Does the moon also pull on Earth? If so, which pull is stronger?
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The girls will win. The force of friction is greater between the girls' feet and the floor than between the boys' feet and the floor. When both the girls and the boys exert action forces on the floor, the floor exerts a greater reaction force on the girls' feet. The girls stay at rest and the boys slide toward the girls.
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A tug of war occurs between boys and girls on a polished floor that's somewhat slippery. If the boys are wearing socks and the girls are wearing rubber-soled shoes, who will surely win, and why?
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pair of forces.
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A force interaction requires at least a(n)
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opposite in direction and equal in magnitude at the same time
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Whenever one object exerts a force on a second object, the second object exerts a force on the first that is
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road.
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The force that directly propels a motor scooter along a highway is that provided by the
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move downward.
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When you jump vertically upward, strictly speaking, you cause Earth to
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net force.
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A system undergoes acceleration only when acted on by a(n)
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accelerates.
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If a net force acts on a horse while it is pulling a wagon, the horse
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mass of the dough.
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At a pizza shop, the cook throws the pizza dough in the air. The amount of force the cook exerts on the dough depends on the
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Yes.
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In the interaction between a hammer and the nail it hits, is a force exerted on the nail?
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Yes.
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ON the hammer?
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Two forces, action and reaction.
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How many forces occur in this interaction?
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They are equal and opposite according to Newton's Third Law.
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When a hammer exerts a force on a nail, how does the amount of force compare with that of the nail on the hammer?
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The floor pushes back on your feet.
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When you walk along a floor, what pushes you along?
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The water pushing you forward.
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When swimming, you push the water backward- call this action. What is the reaction force?
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The arrow pushes back on the bowstring.
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If the action is a bowstring acting on an arrow, identify the reaction force.
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The world has such a large mass that is acceleration downwards is negligible.
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When you jump up, the world really does recoil downward. Why can ́t this motion of the world be noticed?
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The forces are the same, equal and opposite.
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When a rifle is fired, how does the size of the force of the rifle on the bullet compare with the force of the bullet on the rifle?
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The rifle will recoil backwards with less acceleration because it has a larger mass. F = M x a
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How does the acceleration of the rifle compare with that of the bullet? Defend your answer.
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The rocket doesn ́t push against the air, the rocket pushes against the hot gases coming out of its thrusters. The hot gases push back propelling the rocket forward.
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How can a rocket be propelled above the atmosphere where there is no air to "push against"?
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One. The orange pulling on the apple.
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In the interaction between an apple and an orange, how many forces are exerted on the apple?
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One, the apple pulling the orange.
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On the orange?
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Yes.
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Are these forces equal in strength?
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Yes.
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Are these forces opposite in direction?
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No. They act on different objects - the apple and the orange.
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Consider the orange system. Do action and reaction forces cancel each other in the orange system?
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Yes, towards the apple.
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Does the orange system accelerate?
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Yes, if you take them both together. One force pulls to the right and another equal and opposite force to the left.
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Consider the orange-apple system. Do action and reaction forces cancel each other in this system
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They remain together.
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Do the orange and apple accelerate away from each other, or do they remain together?
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Two, the horse pulling the cart forward and friction pulling the cart backwards.
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In the horizontal direction, how many forces are exerted on the cart?
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Force of pull of horse forward - Force of friction on cart backwards
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What is the net horizontal force on the cart?
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Two: floor pushing horse forward, and cart pulling back the horse (due to friction of the cart).
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How many horizontal forces are exerted on the horse.
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Ffp - Fpc
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What is the net horizontal force on the horse?
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Two. One on the ground and one on the cart.
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How many horizontal forces are exerted by the horse on the other objects?
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Two: Fp and Ff. The force pulling the system forward and the friction holding the system back.
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How many horizontal forces are exerted on the horse-cart system?
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Fp -Ff
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What is the net horizontal force on the horse-cart system?
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Otherwise the net force would be zero and no acceleration would happen.
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In order to increase its speed, why must the horse push harder against the ground than it pulls on the wagon?
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200 N is exerted back on you.
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If you hit a wall with a force of 200 N, how much force is exerted on you?
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It cannot exert a reaction force of 200 N back on you.
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Why can ́t you hit a feather in midair with a force of 200 N?
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If you give a hug you get one back. If you kiss somebody, they kiss you back. Eye for an eye, tooth for a tooth. If you hit somebody, they hit you back with an equal and opposite force.
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How does the saying "You get what you give" relate to Newton's third law?
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Your body pulling on the earth
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Your weight is the result of the gravitational force of Earth on your body. What is the corresponding reaction force?
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The handlebars push down with the same force as your pull. This force is transmitted to the pedals.
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Why can you exert a greater force on the pedals of a bicycle if you pull up on the handlebars?
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Yes, they are the only forces acting on a non accelerating person; no they are 2 interactions - person-earth and floor-person.
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Consider the two forces acting on a person who stands still, namely the downward pull of gravity and the upward support of the floor. Are these forces equal and opposite? Do they comprise an action-reaction pair? Why or why not?
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You push forward on the log. The log pushes backward on you. This backward force makes the log go backward.
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If you walk on a log that is floating in the water, the log moves backward. Why?
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One can exert a greater horizontal force on a carpet than a polished floor due to friction. So you get a greater reaction force.
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Why is it easier to walk on a carpeted floor than on a smooth, polished floor?
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Yes, but the accelerations produced by these equal forces are quite unequal because of the differences in the masses
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If you step off a ledge, you accelerate noticeable toward Earth because of the gravitational interaction between you and Earth. Does Earth accelerate toward you as well? Explain.
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Action is you pushing down on sink - reaction is sink pushing up on you. This takes some of the burden off of the scale. If the action is you pushing up on the sink, the reaction is the sink pushing down on you. This adds burden to the scale.
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Suppose you're weighing yourself while standing next to the bathroom sink. Using the idea of action and reaction, explain why the scale reading will be less when you push down on top of the sink. Why will the scale reading be more if you pull up on the bottom of the sink?
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The ground pushed upward on the jumper, which is the force that provides an upward accelerate. In the air, only the force of gravity acts on the jumper. Upward acceleration ceases and the acceleration is downward - g
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When a high jumper leaves the ground, what is the source of the upward force that accelerates her? What force acts after her feet are no longer in contact with the ground?
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1000 N exerted on the earth by the communications satellite.
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hat is the reaction force to an action force of 1000 N exerted by the Earth on an orbiting communications satellite?
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It is, but negligibly, due to Earth's huge mass
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If action equals reaction, why isn't Earth pulled into orbit around satellites?
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Impact force is the same. Because of the bicycle's smaller mass, the change in motion is greater for the bicycle
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If a bicycle and a massive truck have a head-on collision, upon which vehicle is the impact force greater? Which vehicle undergoes the greater change in its motion? Defend your answers.
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Magnitude is the same. The bug decelerates more because it has less mass
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A speeding bus makes contact with a bug that spatters onto the windshield. Because of the sudde3n force, the unfortunate bug undergoes a sudden deceleration. Is the corresponding force that the bug exerts against the windshield greater, less or the same? Is the resulting deceleration of the bus greater than, less than or the same as that of the bug?
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The force is that which the exhaust exerts. It is the reaction to the force that the rocket exerts on the exhaust gases
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Some people used to think that a rocket could not travel to the moon because it would have no air to push against once it left Earth's atmosphere. We now know that idea was mistaken. What force propels a rocket when it is in a vacuum?
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No. There is a force applied to the cannonball that causes movement as long as it is greater than all of the other forces acting on the cannonball. That force is equal to the one on the cannon, which will cause the cannon to move if it is greater than the other forces acting on it
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Since the force that acts on a cannonball when a cannon is fired is equal and opposite to the force that acts on the cannon, does this imply a zero net force and therefore the impossibility of an accelerating cannonball?
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200 N; yes; no, the reaction to your push on the refrigerator is the refrigerator's push back on you
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Suppose you exert 200 N on your refrigerator and push it across the kitchen floor at constant velocity. What friction force acts between the refrigerator and the floor? Is the friction force equal and opposite to your 200-N push? Does the friction force make up the reaction force to your push?
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You cannot do it for 2 reasons. 1 you can pull with as much force as your friend can pull back with. 2 scales on different parts of the same rope have to have the same reading because the rope is experiencing tension as a whole everywhere, and it will be the same wherever you measure it.
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Your teacher challenges you and your best friend to each pull on a pair of scales attached to the ends of a horizontal rope,in tug- of-war fashion, so that the readings on the scales will differ. Can this be done? Explain.
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50 N, the same as if the pulls were horizontal
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A pair of 50-N weights is attached to a spring scale as shown. Does the spring scale read 0, 50, or 100 N?
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The same; doubled (if they both pull in the same direction)
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The strong man can withstand the tension force exerted by the two horses pulling in opposite directions. How would the tension compare if only one horse pulled and the left rope was tied to a tree? How would the tension compare if the two horses pulled in the same direction, with the left rope tied to the tree?
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Downward because he pulls as he climbs
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A balloon floats motionless in the air. A balloonist climbs up the supporting cable. In which direction does the balloon move as the balloonist climbs? Explain.
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More than because you accelerate upward as you push
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When you get up from a sitting position, do your feet push against the floor with a force equal to, more than, or less than your weight? Explain.
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More than, because the barbell accelerates upward
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When a weightlifter jerks a barbell over his head, is the force exerted on the barbell more than, less than, or equal to the barbell's weight? Explain
