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Homework Help: Work and Kinetic Energy of cord

  1. Oct 26, 2007 #1
    1. The problem statement, all variables and given/known data
    A cord is used to vertically lower an initially stationary block of mass M at a constant downward acceleration of g/4. When the block has fallen a distance d, 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. [Maintain Significant Figures]

    2. Relevant equations

    3. The attempt at a solution
    I have no idea how to even begin this problem. First of all, I cannot even begin to comprehend how to obtain an answer consisting of actual numbers from a mass "M" and a distance "d". The only physical number I have is g/4 which is 2.45m/s^2. If someone can help me with this problem, it will be greatly appreciated.
  2. jcsd
  3. Oct 26, 2007 #2


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    Homework Helper

    In the absence of the cord, the block will experiance a force = Mg. With cord the force is Mg/4. Hence the cord is exerting a retarding force
    = 3Mg/4. With this retarding force the block has fallen a distance d. Therefore the work done by the cord on the block is = 3Mgd/4, and work done by the gravity = Mgd/4. From this you can find out KE and speed.
    To solve the problems there is no need of numerical values.
  4. Oct 26, 2007 #3
    So my answer will be in the form of "M" and "d"?
  5. Oct 26, 2007 #4


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    Homework Helper

    Yes, it's in the form of M and d. Apologies to rl.bhat, but can you explain why the work done by gravity is NOT Mgd/4? Notice the work done by the cord is taken out of the block and the work done by gravity goes into the block.
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