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Homework Help: Help needed immediately

  1. Aug 11, 2005 #1
    Help needed immediately!!!

    A = 50kg
    B = 100kg
    coefficient of friction = 0.25

    A is moving along the inclined plane for 20m.What is the change of kinetic energy of A if the initial velocity of A is 0?

    Work done by friction force = change in KE. + change in PE

    My lecturer told me to find out the change of KE by using this equation.
    But in my opinion, isn`t the total work done = change in KE. + change in PE ??

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  2. jcsd
  3. Aug 11, 2005 #2
    And how about this?

    mass of the box is 10kg
    It is pulled by a force of 100N in the same direction of the inclined plane. The initial velocity of the box is 1.5m/s

    After calculation,
    work done by gravity = -167.59
    work done by friction = -184.18
    work done by 100N = 500J

    To find out the change in KE, the total work done = change of KE.

    Why shouldn`t the total work done = change of KE. + change of PE. ??

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  4. Aug 11, 2005 #3
    I see what you are saying, however the gain in kinetic energy is equal to the loss in Potential energy, If what you are saying is correct that the workdone is equal to the change of kinetic energy plus the change in potential energy, the workdone will be 0.

    What is happening is if you take the definition of Workdone which is

    See Attachment....

    Hope this helps


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  5. Aug 11, 2005 #4
    I just want to know which solution is correct. Thanks.
  6. Aug 11, 2005 #5
    W=change in kinetic energy is correct.
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