Start Ch - University of...
Transcript of Start Ch - University of...
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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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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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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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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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Question
Can you conserve momentum with a system initially at rest and at the end everything is moving at speed?
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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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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?