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Homework Help: Quick questions on work and energy

  1. Mar 9, 2007 #1
    Can work be done on a system if there is no motion?
    I would say no, no motion = no energy...

    Is it possible for a system to have negative potential energy?
    I would say yes, since the choice of the zero of potential energy is arbitrary.

    A 500-kg elevator is pulled upward with a constant force of 550N for a distance of 50 m. What is the work done by the 550N force?
    From what I understand, we multiply 550N by 50 m, and get about 3.00x 104 J.

    Thanks for your help!
     
  2. jcsd
  3. Mar 9, 2007 #2

    Dick

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    I think you are pretty much correct.
     
  4. Mar 9, 2007 #3

    berkeman

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    I would agree with you on all three. But the first question is thought provoking. I wonder if we can think of a case where there is work done, but no motion. Certainly that is true in cases where there is no *net* motion, like spinning a wheel with friction bearings. But no motion at all....hmmm.

    Chemical energy conversion...is that considered work? I don't think so, but maybe someone else can think of a creative case.
     
  5. Mar 9, 2007 #4
    Thank you!
     
  6. Mar 9, 2007 #5

    AlephZero

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    You can't do mechanical work without motion, but there are other ways to increase the energy of a system - for example adding heat energy, or storing electrical charge in a capacitor. "Increasing the energy" is the same as "doing work".

    Both correct.
     
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