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Start Ch.9Impulse and Momentum

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Impulsive Force

An impulsive force is a large force exerted during a small time. EX: A tennis racket hitting a tennis ball.

Impulsive forces are a function of time, F(t).

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Momentum

Units:

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Impulse

Note: The same force applied for a longer time gives a larger impulse.

Units:

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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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Impulse-Momentum Theorem at work

Demo: Bat, finger, weight

Demo: Pen, hoop

Demo: Cloth pull, 2 videos

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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 & SheetHappy / Sad ball, ramp, wood block

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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?

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Example

A 200g car is moving to the right with speed 6m/s and collides with a 200g car moving to the left with speed 6m/s. The cars stick together. What is their final velocity after the collision?

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ExampleA 200g car is moving to the right with speed 6m/s and collides with a 600g car moving to the left with speed 6m/s. The cars stick together. What is their final velocity after the collision? Which experiences a larger acceleration?

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Demo: Air carts

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The physics of car crashes

What determines your safety in a car crash?

Better to be in a small vehicle or large vehicle?

Better to be in a heavy vehicle or a light vehicle?

Better to use seat belts / air bags?

Better to be in an old vehicle or new vehicle?

Videos: 30 minutes of videos (long = 23min)

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Official US testers

Insurance Institute of Highway Safety (IIHS)www.iihs.org

National Highway Traffic Safety Administration (NHTSA)

www.safercar.gov

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ExplosionsA rifle shoots a .338Lapua Magnum bullet. The rifle weighs 9lbs. The bullet weighs 19.44g. The bullet leaves the gun at 853m/s. How fast does the rifle recoil?

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Video

Any ways to cheat physics?Is there a way to shoot the same bullet out of the same gun with much less recoil?

Yes!

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Muzzle Brakes

Videos

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Question

Can you conserve momentum with a system initially at rest and at the end everything is moving at speed?

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This is how thrust works

Demo: Wagon

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Momentum in 2-D

Treat each direction separately.

Conservation of momentum holds for any direction as long as there is no net force in that direction.

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Law of Conservation of Mass

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ExampleA 10g projectile is traveling east at 2m/s when it suddenly explodes into three pieces. A 3g fragment is shot due west at 10m/s while another 3g fragment travels 40° N of E at 12m/s. What are the speed and direction of the third fragment?

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End Ch.9