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Energy Required to Lift a Heavy Box

  1. Mar 11, 2005 #1
    As you are trying to move a heavy box of mass [tex]m[/tex], you realize that it is too heavy for you to lift by yourself. There is no one around to help, so you attach an ideal pulley to the box and a massless rope to the ceiling, which you wrap around the pulley. You pull up on the rope to lift the box.

    A.) What is the magnitude [tex]F[/tex] of the upward force you must apply to the rope to start raising the box with constant velocity?
    Express the magnitude of the force in terms of [tex]m[/tex], the mass of the box.

    I think the answer should be mg/2

    is this correct?
  2. jcsd
  3. Mar 11, 2005 #2

    Doc Al

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    Staff: Mentor

    Yes. The pulley gives you a mechanical advantage, reducing the force (but not the energy!) needed to lift the box.
  4. Mar 11, 2005 #3

    James R

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    The pulley won't make it any easier. The force you'll need to apply with a single pulley is still F=mg.


    Edit: Hmm... I think I might have mistaken the way the pulley is connected, in which case the force may be mg/2. A diagram would be nice!
  5. Mar 12, 2005 #4
    With a single pulley the way you described it there's no way to shorten the distance over which the force is applied (it is the same as the distance over which the pulley rises). So the force is still mg. The only facilitation is that you apply the force downward.
  6. Mar 12, 2005 #5

    Doc Al

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    Please reread the original post: The pulley is attached to the box, not the ceiling.
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