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Third Law of Motion

Section 3.4

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+Rockets!

What happens when a rocket takes off?

How does it get propelled into the sky?

What does this have to do with Newton’s Laws?

Why does the shuttle system move in the opposite direction of the gases?

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+Equal and Opposite Forces

What happens when two tennis balls collide?

Like with the shuttle, the gases from the exhaust tanks create a downward force that propel the rocket upward.

Newton’s Third Law of Motion- When an object exerts a force upon a second object, the second object exerts an equal and opposite force upon the first object. Every action has an equal and opposite reaction.

http://quest.nasa.gov/space/teachers/rockets/images/skateboard.gif

+Forces and Newton’s Third Law

Look at Figure 3.14 on page 71 in your textbook: What happens when the diver jumps down on the diving

board? What is the action force? What is the reaction force? How will the force of the diving board affect the diver’s

performance?

We know that action and reaction forces are equal and opposite, therefore, why don’t they cancel each other out?

This is because the forces act on different objects, therefore they don’t cancel each other out. What is the action force acting on? What is the reaction force acting on?

http://study.com/cimages/multimages/16/board_jumper.jpg

+Observing Newton’s Third Law

In Figure 3.15 on page 71, how is the boat moving?

The boat is moving because the oars and water are using each other to repel each other.

When an oar is put into the water the water exerts and equal force on both sides.

When the oars are pulled, the oars push against the water, and the water pushes the oars back with and equal and opposite force.

Therefore, which way is the boat moving relative to the oars?

http://study.com/cimages/multimages/16/rowing_boat.jpg

+What have we learned?

What is Newton’s Third Law of Motion?

Explain how action and reaction forces are important to a tennis game?

What are the action and reaction forces acting when someone jumps from a canoe into a riverbank? Explain why the jumper often falls into the water.

Explain how Newton’s third law of motion is at work when you walk.

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