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Unit 4: Momentum and Energy Chap. 7 Momentum
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Which is harder to stop, a truck traveling at 55 mi/hr or a small car traveling at 55 mi/hr? Why?
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Which is harder to stop, a slow moving large truck or a very fast moving small car? Why?
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What is MOMENTUM? A. Momentum: inertia in motion B. Momentum = mass x velocity (or speed) p = m. v C. Momentum is greater if it has 1. high speed 2. large mass 3. or both
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What could you hypothesize if the mass of an object remained the same (same object), but the momentum changed? Acceleration occurs (velocity increases)
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What causes accelerations? Force How do you get a large acceleration? large force or apply a smaller force for a longer period of time
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Impulse Impulse: force over a period of time Impulse = force x time I = F. t The higher the impulse, the greater the change in momentum
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Impulse The higher the impulse, the greater the change in momentum. I = p F. t = (m. v)
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Would you rather hit a brick wall or a haystack? Why It has to do with impulse. The impulse would be the same BUT the force and the time that make up the impulse are not the same. You would be in contact with the haystack longer than you would be in contact with the brick wall.
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brick wall same impulsehaystack t. F = I= t. F t. F = I= t. F Therefore, the force of the brick wall would be greater than the force of the haystack.
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If you jump off a lab table, should you hit the ground with your legs straight or should you bend your legs? Why?
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Which is better, to drop a ceramic dish on the sidewalk, or on thick carpet? Why?
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Which would be safer, a padded dashboard or a metal dashboard? Why?
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HOW IS BOUNCING DIFFERENT? Impulse is greater when something bounces. There are really 2 steps when bouncing: 1. impulse to bring momentum to a stop 2. provide additional impulse to bring it back up
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HOW IS BOUNCING DIFFERENT? The impact force will still react the same. 1. If the impulse is over a long time, then the impact force is small. Example - a circus net 2. If the impulse is over a short time, then the impact force is large. Example - a body dumped from a 10 story window
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What happens to momentum during collisions? Conservation of Momentum Total momentum before equals total momentum after (when there are NO outside forces acting). In the absence of an external force, the momentum of a system remains unchanged.
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How do we know? Newton’s 3rd Law: for every action, there is an equal and opposite reaction.
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Types of Collisions ELASTIC 1. Objects hit but don’t stick together 2. Transfers momentum INELASTIC 1. Objects hit and stick together 2. Shares momentum
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