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Work done by gravity problem

  1. Apr 10, 2013 #1
    1. The problem statement, all variables and given/known data
    An object of mass m is moved straight up by a distance h. The work that gravity does on the object is:
    (a) -mh
    (b) +mh
    (c) -mgh
    (d) +mgh
    (e) None of the Above

    2. Relevant equations
    Work = ∫F dl
    PE = mgh

    3. The attempt at a solution
    I have a feeling the answer is (c) -mgh because work is done to lift the object. The object is gaining potential energy so it's gaining +mgh and due to Newton's 3rd law, the work done by gravity is -mgh. Is that right?
  2. jcsd
  3. Apr 10, 2013 #2


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    Staff Emeritus
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    Yes, it's answer (c) . But you can get it directly from the definition of work.

    What is the direction of the displacement?

    What is the direction of the gravitational force?
  4. Apr 10, 2013 #3
    Ah, right, hadn't thought of that. The direction of the displacement is opposite the direction of the force. Thank you.
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