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Homework Help: What is work done by gravity when lifting an object?

  1. Oct 12, 2011 #1
    1. The problem statement, all variables and given/known data
    I have a brick of mass m in my hand. I move the brick vertically up h meters.


    2. Relevant equations

    What is the Work done to the brick by gravity?

    3. The attempt at a solution
    Work = Fd The F for my hand on the brick is mg going up and d is h so the work done by my hand to the brick is mgh upward. But now what is the work done by gravity? This concept is still blurry to me. Is Wgravity=mgh downward?
     
  2. jcsd
  3. Oct 12, 2011 #2

    Doc Al

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    Staff: Mentor

    Work is still just force X displacement. Since the force of gravity is downward, while the displacement is upward, what would be the sign of the work done by gravity?
     
  4. Oct 12, 2011 #3
    I thought that work can only be done by a force that is in the same direction as the displacement. So wouldn't the work done just be 0? Or is that just a simple matter of how you define your system?
     
  5. Oct 12, 2011 #4

    Doc Al

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    Staff: Mentor

    No. Work can only be done by a force that has a component in the direction of the displacement; that component may well be negative.
     
  6. Oct 12, 2011 #5
    Roger that.
     
  7. Oct 12, 2011 #6
    So gravity is down. So is Wgravity= -mgh?
     
  8. Oct 12, 2011 #7

    Doc Al

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    Staff: Mentor

    Right. The work you do (via your hand) is +mgh; the work done by gravity is -mgh.
     
  9. Oct 13, 2011 #8
    that can't be right though can it? That means that in lifting it up there is no net work done.
     
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