Lec5 - Today - Chapter 6: Momentum Next week: Tuesday I am...

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Next week: Tuesday – I am away at a conference so Mr. Arun Rajam (PhD student in physics) will cover this class. Friday – No class, due to Rosh Hoshanah (Hunter closed) Today -- Chapter 6: Momentum
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Today -- Chapter 6: Momentum Momentum = “inertia in motion” Specifically, momentum = mass x velocity = m v Eg. Just as a truck and a roller skate have different inertia, when they are moving, they (generally) have different momenta. Question: (i) Does the truck always have more inertia than the roller skate? (ii) What about momentum? (i) Yes (mass larger) (ii) No – eg a roller skate rolling has more momentum than stationary truck. Momentum depends on speed as well as mass .
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Impulse How can the momentum of an object be changed? By changing its mass, or, more usually, its velocity – i.e. by causing an acceleration. What causes acceleration? A force But the time over which the force acts, is also important. Eg. If trying to get a broken down car moving, and you push tremendously but only for a split-second, it won’t move. You need to exert the force for a longer time.
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Impulse cont. The “effectiveness” of the force in causing a change in momentum is called the impulse: Impulse = force x time interval = F t How exactly is the momentum changed? Use N’s 2 nd law, a = F/m , or, F =ma. So, impulse = ma t = m (change in velocity/time) time = m (change in velocity) i.e. impulse = change in momentum Ft = ( mv)
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Using the impulse-momentum relation Increasing momentum As highlighted by the broken-down car example, need to apply large force for a large time. Eg. The longer the barrel of a cannon, the greater the speed of the emerging cannonball because the forces on it from the expanding gasses have more time to act. Eg. Why does an archer pull his arrow all the way back before releasing it? To give more time for the (time-varying) elastic force of the bow to act, so imparting greater momentum.
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Decreasing momentum over a long time Often you want to reduce the momentum of an object to zero but with minimal impact force (or injury). try to maximize the time of the interaction (recall Ft = ( momentum)) Eg. Riding with the punch, when boxing, rather than moving into it… Here, by moving away, the time of contact is extended, so force is less than if he hadn’t moved. Here, by moving into the glove, he is lessening the time of contact, leading to a greater force, a bigger ouch!
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Eg. Car crash on a highway, where there’s either a concrete wall or a barbed-wire fence to crash into. Which to choose? Naturally, the wire fence – your momentum will be decreased by the same amount, so the impulse to stop you is the same, but with the wire fence, you extend the time of impact, so decrease the force. i.e. same
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This note was uploaded on 07/18/2011 for the course PHYS 10 taught by Professor Goldberg during the Fall '11 term at CUNY Hunter.

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Lec5 - Today - Chapter 6: Momentum Next week: Tuesday I am...

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