Newtons Third Law of Motion

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Newtons Third Law of Motion
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• Newtons Third Law in ACTION!
• More Action!!!!

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Newtons Third Law of Motion
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• Whenever one object exerts a force on a second
  object, the second object exerts an equal and
  opposite force on the first.

What will happen if two ice skaters push off each
other? Of course -- they both go backwards!
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Newtons Third Law of Motion
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• Another way to write Newtons 3rd Law
• - Every action has an equal and opposite
  reaction!

This building may be pushing on the ground, but
the ground is also pushing back up on the
building!
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• One example of Newtons 3rd Law is holding a fire
  hose

It is very difficult to hold a fire hose. The
force of the water is very strong, and you must
apply an equal force against the hose. Imagine
what would happen if you were standing on ice and
then turned on the water Your momentum would be
equal and opposite to the momentum of the water!!!
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Newtons Third Law is called the Law of Momentum.
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Momentum a measure of how hard it is to change
the motion of a moving object.
What do you think that depends on?
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MOMENTUM
The momentum of objects depends on their mass and
their velocity.
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The 3rd Law - momentum
If something is not moving, then it will have
zero momentum.
Dang, I have no momentum.
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MOMENTUM
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Here is how we calculate momentum Momentum
mass x velocity
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MOMENTUM Newtons 3rd Law
What is the momentum of a 0.15 kg billiard ball
that is moving 2 m/s? (0.15 kg) x (2 m/s) 0.3
kgm/s (weird unit!)
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MOMENTUM
When objects run into each other or push off of
each other, the total momentum of all the objects
involved has to stay the same. This is called
The Law of Conservation of Momentum.
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Another Example of the 3rd Law
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• A Water Rocket !

When water is expelled from the bottom of the
rocket, it takes off. Why? The momentum of the
rocket equals the momentum of the water!
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Newtons Third Law of Motion
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• The rocket pushes on the water, and the

You should draw this!!!
The momentum of the ACTION and REACTION are
equal.
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• Momentum mass x velocity

• The momentum of the rocket the momentum of the
  water.
• mass x velocity mass x velocity
• (of rocket) (of rocket) (of water)
  (of water)

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Newtons Third Law of Motion
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Weeee!
If you want to move in space, you must throw or
shoot something away from yourself!
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Newtons Third Law of Motion
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Go away, silly Earthling!
If you throw off, push off of, or shoot something
in space, you will be pushed backward. Your
momentum will equal the momentum of the other
thing (or alien!).