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Kinetic/potential energy with a pulley system

  1. Oct 27, 2012 #1
    1. The problem statement, all variables and given/known data
    This is the simplest example:


    But I have to generalize with more pulley things on it. How does the pulley affect the kinetic energy of the car on the air track, and the potential energy of the block pulling the car?

    2. Relevant equations
    Nothing quantitative

    3. The attempt at a solution
    When I conducted this experiment, I noticed that the potential energy of the block and the kinetic energy of the car did not match up in the end, so I'm thinking the pulleys had to do something with it (it used two of them). Does some of the energy get transferred to the pulley system?
  2. jcsd
  3. Oct 27, 2012 #2
    What were your numbers?
  4. Oct 27, 2012 #3
    The potential for the weights were just about the same as kinetic for the car, but there was a slight discrepancy so it might've just been error but I'm inclined to intuitively believe that if there were more pulley things, that the numbers might've been different
  5. Oct 27, 2012 #4


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    "more pulley things" is rather vague. There are all sorts of ways more pulleys could be introduced. But the generality is that real pulleys have mass and hence moment of inertia. Some energy will go into their rotation, and you won't get that back. Any chance you could calculate that?
  6. Oct 27, 2012 #5
    I don't think I have the proper equipment for that, and also I haven't really learned about how energy works in that way. But I meant more pulleys like in:


    or something similar. With the same direction of motion for the weights and everything
  7. Oct 28, 2012 #6


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    If you have two identical pulleys, and in one experiment you only use one, while in the other you have them in series (as in your diagram), you might be able to work out how much energy is going into them. It won't be straightforward because when there are two the top speed will be less, so each pulley will take less energy than the one by itself. But in principle it would allow you to calculate what they take.
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