Physics Ch. 5

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When you push against a wall with your fingers, they bend. Identify the force or forces involved.

You exert an action force and the wall exerts an equal and opposite reaction force.

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?

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.

How many forces are required for an interaction?

Two forces, an action and a reaction, are needed for an interaction.

State Newton’s third law of motion.

For every action there is an equal and opposite reaction.

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.

The reaction force is the force by the ball on the bat.

If the system of Figure 5.9 is only the orange, is there a net force on the system when the apple pulls?

Yes, there is the action of the apple on the orange.

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)?

The net force is zero because the action and reaction forces are equal and opposite.

To produce a net force on a system, must there be an externally applied net force?

Yes, there must be an external net force on the system.

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?

If you kick it, there is a net force; if both of you kick it, there is not.

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?

Yes, you pull up on Earth with the same force.

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?

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.

Identify the force that propels a rocket.

The rocket is propelled by the reaction force from the particles accelerated out the rear by an action force from the rocket.

How does a helicopter get its lifting force?

The helicopter exerts downward forces on air; the reaction forces of the air on the helicopter are upward and called lift.

Can you physically touch a person without that person touching you with the same amount of force?

When you touch a person, they must touch you with an equal and opposite force.

What is meant by the term vector resolution?

A vector can be broken into two components at right angles that add together to make the original vector.

What happens to the magnitude of the normal vector on a block resting on an incline when the angle of the incline increases?

The magnitude of the normal vector decreases.

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?

It is equal and opposite.

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?

The vertical component decreases in magnitude until it reaches the maximum height and then increases; the horizontal component is constant.

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 ____.

Inertia, acceleration, action -reaction

Which of Newton’s three laws focuses on interactions?

Newton’s third law

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