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Calculating work not given acceleration, work done by gravity

  1. Apr 18, 2014 #1
    1. The problem statement, all variables and given/known data
    A rope lifts a mass of 20kg vertically 80cm.

    How much work is done by the rope on the mass?

    How much work is done by gravity on the mass?



    2. Relevant equations
    W = FΔd
    Eg = mgh


    3. The attempt at a solution
    How much work is done by the rope on the mass?
    W = FΔd
    W = F(0.8)

    This is as far as I've gotten. I don't think this is the correct solution. For this to work, I would need to put the applied force of the rope. The problem is that I don't know the acceleration of the rope, so I'm thinking this may be the completely wrong approach.

    How much work is done by gravity on the mass?
    So far what I have is:
    W = FΔd
    W = mgΔd
    W = (20)(9.81)(0)
    W = 0

    Because gravity doesn't move the mass, the work done by gravity is zero. I believe this is the correct solution.
     
  2. jcsd
  3. Apr 18, 2014 #2

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    It is not.

    It does not matter "what" moves the mass. The mass moves against the direction of the gravitational force, that needs work. Energy is conserved, so where does that work come from?

    The speed of the process and other things don't matter, you have the correct formulas there.
     
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