060225_notes512questions

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23:36:39 We put energy into the rubber-band-and-rail system, in the form of the work we do stretching the rubber band. In detail, what happens to this energy from the instant we start pulling back to the instant the rail stops?

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RESPONSE --> The energy is PE. When released from being help the PE decreases which increases the KE

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23:37:15 The system stores most of the energy as elastic PE of the rubber band. When the system is released the PE decreases and goes into the KE of the system. There are also thermodynamic effects. As the rubber band is stretched the thermal energy that results from the stretching is released into the air. When the rubber band becomes unstretched it cools off, but the thermal energy that was released does not return to the rubber band.

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RESPONSE --> there is thermodynamic effects. this is released in the air and does not return

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23:38:04 What happens to the KE of the rail as it slides across the floor? Where does it go?

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RESPONSE --> KE increases which is lost to friction.

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23:38:09 ** The KE was gained at the expense of the rubber band's PE, but as it slides across the floor it is lost to friction. The result of friction is thermal energy, or heat, which is dissipated from the rail and the surface. **

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RESPONSE --> ok

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23:38:42 From the point of view of the rail, is it doing positive work or negative work as it slides across the floor?

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RESPONSE --> positive work

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23:38:54 ** It's doing positive work, but the reason is that it is exerting a force against friction, and this force is in the direction of its motion. **

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RESPONSE --> it is working against friction

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23:42:01 Does the rubber band supply more or less energy to the rail than the energy we put into it, and where does the difference go?

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RESPONSE --> rubber band supply less energy to the rail than the energy we put into it. The difference goes out against the friction

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23:42:15 When we pull the rail back, we do work, most (but not all) of which will be recovered after we release the rail. This work will therefore be present in the rail in the form of energy of motion. Before we release the rail, this work is potentially there; the potential will become reality after we release the rail. At the instant the rail reaches its original position the rubber band ceases to accelerate it, and it has its maximum energy of motion. As the rail slides across the floor, this energy is dissipated in the form of work done against friction. This continues until all the energy is dissipated and the rail stops.

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RESPONSE --> right does this till the rail stops

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23:43:05 Does the rubber band exert more or less average force when it accelerates the rail than when it was pulled back?

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RESPONSE --> it can't be more because it doesn;t have any more than it can give

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23:43:09 ** It can't exert more average force because if it did it would release more energy than was put into it. **

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RESPONSE --> right

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23:43:47 Is a cooler rubber band stiffer or less stiff than a warmer one?

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RESPONSE --> cooler is less stiff than warmer

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23:43:54 The cooler rubber band is less stiff than the warmer one, contrary to our intuition about how things stiffen when they are cooled.

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RESPONSE --> right

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Very good work. Let me know if you have questions.