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I don't understand why this is wrong (Work and kinetic energy problem)

  1. Feb 22, 2006 #1
    Okay, here's the problem:

    A cord is used to vertically lower an initially stationary block of mass M = 6.4 kg at a constant downward acceleration of g/2. When the block has fallen a distance d = 4.8 m, find (a) the work done by the cord's force on the block, (b) the work done by the gravitational force on the block, (c) the kinetic energy of the block, and (d) the speed of the block. (Note : Take the downward direction positive)

    For a I come up with the equation:

    F - Mg = Mg/2.

    Solving for F I get:

    F = 3Mg/2

    Plugigng this into the basic equation for work I get:

    W = 3Mgd/2

    But when I plug in my values for M, g, and d, I get a wrong answer. Why?
     
  2. jcsd
  3. Feb 22, 2006 #2

    nrqed

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    If there is a *downward* acceleration of g/2, it means that [itex] a_y = - g/2 [/itex]. You used +g/2 in your equation.
     
  4. Feb 22, 2006 #3
    But it says to take the downward direction to be positive.
     
  5. Feb 22, 2006 #4

    nrqed

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    All right. Then your equation should be -F + Mg = Mg/2
     
  6. Feb 23, 2006 #5
    And then when you calculate the work done, F is upward and d is downward.

    -Dan
     
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