# Newton’s third law of motionPage 4

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Newton’s Third Law of Motion

Consider the same system, but with external force of friction on it.

Same internal action and reaction forces (between the orange and apple) cancel.

A second pair of action-reaction forces (between the apple’s feet and the floor) exists.

Slide 24

Newton’s Third Law of Motion

One of these acts by the system (apple on the floor) and the other acts on the system (floor on the apple).

External frictional force of floor pushes on the system, which accelerates.

Second pair of action and reaction forces do not cancel.

Slide 25

When lift equals the weight of a helicopter, the helicopter

A. climbs down.

climbs up.

hovers in midair.

None of the above.

Newton’s Third Law

Slide 26

When lift equals the weight of a helicopter, the helicopter

A. climbs down.

climbs up.

hovers in midair.

None of the above.

Newton’s Third Law

Slide 27

When lift is greater, the helicopter

A. climbs down.

climbs up.

hovers in midair.

None of the above.

Newton’s Third Law

Slide 28

When lift is greater, the helicopter

A. climbs down.

climbs up.

hovers in midair.

None of the above.

Newton’s Third Law

Slide 29

A bird flies by

A. flapping its wings.

pushing air down so that the air pushes it upward.

hovering in midair.

inhaling and exhaling air.

Newton’s Third Law

Slide 30

A birds flies by

A. flapping its wings.

pushing air down so that the air pushes it upward.

hovering in midair.

inhaling and exhaling air.

Newton’s Third Law

Slide 31

## Summary of Newton’s Three Laws of Motion

Newton’s first law of motion (the law of inertia)

An object at rest tends to remain at rest; an object in motion tends to remain in motion at constant speed along a straight-line path.

Newton’s second law of motion (the law of acceleration)

When a net force acts on an object, the object will accelerate. The acceleration is directly proportional to the net force and inversely proportional to the mass.

Newton’s third law of motion (the law of action and reaction)

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