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Impulse-Momentum Theorem

An impulse, J, is equal to the change in momentum of an object.

You can have a large force, but if the applied time is very small you get only a small change in momentum!

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Example

A 150g baseball is thrown with a speed of 20m/s. It is hit straight back toward the pitcher at a speed of 40m/s. If the bat is in contact with the ball for 6ms, what was the average force on the ball?

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Impulse Approximation

Other forces often act on an object during a collision.Ex: In the previous example gravity also acted during the collision.

If these forces are very small in comparison to the impulsive force, they can be neglected.

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Stopping an object

Pretend you have an object moving with initial momentum mv and you want it to come to a stop. The impulse-momentum theorem says:

Demo: Egg & Sheet

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Air Bags & Seat Belts

Air bags and seat belts are designed to increase the time it takes for you to come to a stop so that forces on you are less.

See Ch.6 #36

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Newton’s Second Law

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System of particles

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System of particles

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System of particles

What if there are no net external forces?

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Law of Conservation of Momentum

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What is an isolated system?

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Perfectly Inelastic Collisions

Perfectly Inelastic Collision: Two objects collide and stick together.

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Example

A 200g car is moving to the right with speed 6m/s and collides with a 200g car initially not moving. The cars stick together. What is their final velocity after the collision?