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Potential Energy and WET (Incline with friction)

  1. Oct 17, 2008 #1
    A 6.00 kg block is set into motion up an inclined plane with an initial speed of v0 = 8.50 m/s (Fig. P7.23). The block comes to rest after traveling 3.00 m along the plane, which is inclined at an angle of 30.0° to the horizontal.

    [​IMG]

    (a) Determine the friction force exerted on the block (assumed to be constant).
    (b) What is the coefficient of kinetic friction?

    Wfk + Wg = -.5mvi2
    (fk)(x) + (m)(g)(h) = (-.5)(m)(vi2)
    (fk)(3) + (6)(9.8)(3sin30) = (-.5)(6)(8.5^2)
    fk = -101.65

    Final answer is incorrect. Cannot move on to (b) without understanding (a). Any ideas?
     
  2. jcsd
  3. Oct 17, 2008 #2

    LowlyPion

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    Looks to me like the wrong sign.

    KE = PE + Wf
     
  4. Oct 17, 2008 #3
    Tried positive 101.65; the answer is still incorrect.
     
  5. Oct 17, 2008 #4

    LowlyPion

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    When you change the sign there you don't get the same answer. That was my point.
     
  6. Oct 17, 2008 #5
    The change in KE is negative though.
     
  7. Oct 17, 2008 #6

    LowlyPion

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    It's the conservation of Energy.

    Where does the KE go?

    It goes into PE and to work done by friction.

    KE = PE + Wf
     
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