Physics Ch. 5
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When you push against a wall with your fingers, they bend. Identify the force or forces involved.
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You exert an action force and the wall exerts an equal and opposite reaction force.
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A boxer can hit a heavy bag with great force. Why can't he hit a piece of tissue paper in midair with the same amount of force?
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The boxer can only hit the tissue paper with a force as large as the tissue paper can exert on the boxer, and the low-mass tissue can only exert a weak force.
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How many forces are required for an interaction?
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Two forces, an action and a reaction, are needed for an interaction.
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State Newton's third law of motion.
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For every action there is an equal and opposite reaction.
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Consider hitting a baseball with a bat. If we call the force on the bat against the ball the action force, identify the reaction force.
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The reaction force is the force by the ball on the bat.
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If the system of Figure 5.9 is only the orange, is there a net force on the system when the apple pulls?
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Yes, there is the action of the apple on the orange.
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If the system is considered to be the apple and the orange together (Figure 5.10), is there a net force on the system when the apple pulls (ignoring friction with the floor)?
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The net force is zero because the action and reaction forces are equal and opposite.
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To produce a net force on a system, must there be an externally applied net force?
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Yes, there must be an external net force on the system.
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Consider the system of a single football. If you kick it, is there a net force to accelerate the system? If a friend kicks it at the same time with an equal and opposite force, is there a net force to accelerate the system?
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If you kick it, there is a net force; if both of you kick it, there is not.
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Earth pulls down on you with a gravitational force that you call your weight. Do you pull up on Earth with the same amount of force?
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Yes, you pull up on Earth with the same force.
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If the forces that act on a cannonball and the recoiling cannon from which it is fired are equal in magnitude, why do the cannonball and cannon have very different accelerations?
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Remember F = ma, and note that the cannon has a much greater mass than the cannon ball, so the cannon accelerates less for the same force.
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Identify the force that propels a rocket.
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The rocket is propelled by the reaction force from the particles accelerated out the rear by an action force from the rocket.
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How does a helicopter get its lifting force?
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The helicopter exerts downward forces on air; the reaction forces of the air on the helicopter are upward and called lift.
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Can you physically touch a person without that person touching you with the same amount of force?
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When you touch a person, they must touch you with an equal and opposite force.
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What is meant by the term vector resolution?
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A vector can be broken into two components at right angles that add together to make the original vector.
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What happens to the magnitude of the normal vector on a block resting on an incline when the angle of the incline increases?
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The magnitude of the normal vector decreases.
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What is the force of friction acting on a shoe at rest on an incline compared with the resultant of the vectors mg and N?
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It is equal and opposite.
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How does the magnitude of the vertical component of velocity for a ball tossed at an upward angle change as the ball travels upward? How about the horizontal component of velocity when air resistance is negligible?
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The vertical component decreases in magnitude until it reaches the maximum height and then increases; the horizontal component is constant.
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Fill in the blanks: Newton's first law is often called the law of ____; Newton's second law is the law of ____; and Newton's third law is the law of ____.
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Inertia, acceleration, action -reaction
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Which of Newton's three laws focuses on interactions?
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Newton's third law
