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What is Negative Work?

  1. Jul 11, 2012 #1
    In my text book there is given an example of a person holding a chair for three minutes. The persons arm gets tired but no work is done because there is no displacement.

    In the sidebar of my textbook, there is this note:

    "We can calculate the work done by a force on an object, but that force is not necessarily the cause of the obect's displacement. For example, if you lift an object, (negative) work is done on the object by the gravitational force, although gravity is not the cause of the object moving upward!"

    So if in the case of the person holding up a chair, if gravity is doing negative work, then for there to be net zero work done, there must be some positive work done by the person.

    So how can they say there is no work done here?
     
  2. jcsd
  3. Jul 11, 2012 #2
    What is the textbook?
     
  4. Jul 11, 2012 #3
    Physics for Scientists and Engineers 8th edition, by Serway and Jewett
     
  5. Jul 11, 2012 #4

    Doc Al

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    Staff: Mentor

    Gravity only does negative work when you lift the object. Negative work just means that the force and the displacement act in opposite directions.
    Work requires a displacement. If you just hold up a chair there is no displacement, so neither gravity nor the upward force you exert do any work on the chair.
     
  6. Jul 11, 2012 #5
    oh ok, thats pretty obvious now, thanks
     
  7. Jul 11, 2012 #6
    Gravity does no work on the chair held in the same position for 3 minutes. You can hang the chair on a hook rather than holding it. Gravity does no work in either case.
    The side note does not apply directly to your problem.
    The energy spent by our body when applying a force on something in a fixed position was discussed several times.
    It is related to the way the muscles work.

    Edit. Sorry, I was too late.
     
  8. Jul 13, 2012 #7

    Claude Bile

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    Science Advisor

    For gravity to do negative work, the force must be opposite to the displacement.

    So yes, to counter gravity doing negative work, there must be someone pushing against gravity!

    Claude.
     
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