Newton’s third law of motion

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Consider a high-speed bus colliding head-on with an innocent bug. The force of impact splatters the unfortunate bug over the windshield. Which is greater, the force on the bug or the force on the bus?

A. Bug

Bus

Both are the same.

Cannot say

Newton’s Third Law

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Slide 17

Consider a high-speed bus colliding head-on with an innocent bug. The force of impact splatters the unfortunate bug over the windshield. Which is greater, the force on the bug or the force on the bus?

A. Bug

Bus

Both are the same.

Cannot say

Newton’s Third Law

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Comment:

Although the forces are equal in magnitude, the effects are very different. Do you know why?

Slide 18

Two people of equal mass on slippery ice push off from each other. Will both move at the same speed in opposite directions?

A. Yes

Yes, but only if both push equally

No

No, unless acceleration occurs

Newton’s Third Law of Motion

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Slide 19

Two people of equal mass on slippery ice push off from each other. Will both move at the same speed in opposite directions?

A. Yes

Yes, but only if both push equally

No

No, unless acceleration occurs

Explanation:

However they push, the result is equal-magnitude forces on equal masses, which produces equal accelerations; therefore, there are equal changes in speed.

Newton’s Third Law of Motion

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Slide 20

Since action and reaction forces are equal and opposite, why don’t they cancel to zero?

Defining Your System

Consider a single enclosed orange.

Applied external force causes the orange to accelerate in accord with Newton’s second law.

Action and reaction pair of forces is not shown.

Slide 21

Newton’s Third Law of Motion

Consider the orange and the apple pulling on it.

Action and reaction do not cancel (because they act on different things).

External force by apple accelerates the orange.

Slide 22

Newton’s Third Law of Motion

Consider a system comprised of both the orange and the apple

The apple is no longer external to the system.

Force pair is internal to system, which doesn’t cause acceleration.

Action and reaction within the system cancel.

With no external forces, there is no acceleration of system.

Slide 23