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Homework Help: Need Fast Answer.

  1. Mar 1, 2007 #1
    A wooden block with mass 1.50 kg is placed against a compressed spring at the bottom of an incline of slope 30.0 degrees (point A). When the spring is realeased, it projects the block up the inline. At point B, a distance of 6.00 m up the incline from A, the block is moving up the incline at 7.00 m/s and is no longer in contact with the spring. The coefficient of kinetic friction between the block and incline is μk = 0.50. The mass of the spring is negligible. Calculate the amount of potential energy that was initially stored in the spring. Answer at the back of the book is 119 J.
    This question is from University Physics 11th edition ques. 7.73 from Chapter 7.
    I have been trying to do this question for like 3 hours and is sarting to piss me off:mad: . I am using the formula:

    (PEb + PEs + KE)i + Wother = (PEb + PEs + KE)f
    PEb= pot. enery. of block
    PEs =pot. energy of spring
    Woher= work other
    i= initial
    f= final
    Since there is no kE initial i said its 0 and since there is PEb & PEs final i said its 0. So I hae
    mgxsintheta + PEs + μmgcostheta = (1/2) mv^2
    Plugged in values and tried to solve for PEs but can't get 119J. I have simiar problem due (with the values different) that is due in about a hour and half. So need help fast!
  2. jcsd
  3. Mar 1, 2007 #2

    Doc Al

    User Avatar

    Staff: Mentor

    That's OK, as long as you realize that that Wother (the work done by friction) is negative--friction reduces the total mechanical energy.
    If you call the initial PEb = 0, then the final PEb cannot be zero; if you call the final PEb = 0, then the initial PEb will be negative. The final spring PE will be zero.
    Redo this equation in the light of my comments above.
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