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Homework Help: Change in Kinetic Energy of block

  1. Nov 17, 2007 #1
    A 4.5 kg block is set into motion up an inclined plane with an initial speed of 8.9 m/s. The block comes to a rest after traveling 6 m along the plane, which is inclined at an angle of 24 to the horizontal. Determine the magnitude of the change in the block's kinetic energy.

    I tried Delta K= Work of the Force
    so..

    Delta K = mgh but I got it wrong.
     
  2. jcsd
  3. Nov 17, 2007 #2

    rl.bhat

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    Delta K = mgh
    What is the value of h you have substituted in this equation?
     
  4. Nov 17, 2007 #3

    Doc Al

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    You don't need any such equation to answer the question. Instead: What's the initial KE? What's the final KE? What's the change?
     
  5. Nov 17, 2007 #4
    I did also try Delta K = 1/2mVf^2 - 1/2 mVi^2. The final is 0 since it is at rest and I got -178.222 for the initial. But that was also wrong
     
  6. Nov 17, 2007 #5
    for h I user 6sin24.
     
  7. Nov 17, 2007 #6

    Doc Al

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    The initial KE = + 178.22 J. The change in KE is negative. But note that they ask for the magnitude of the change. (So drop the negative sign.)
     
  8. Nov 17, 2007 #7
    ok, I get that now. Magnitude is just the number and not the sign.

    But for the next part it says that (Assumed: The fristional force is constant. Determine the magnitude of the frictional force exerted on the block.)

    do I use, Wf = DeltaK + DeltaU?
     
  9. Nov 17, 2007 #8

    Doc Al

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    That will work.
     
  10. Nov 17, 2007 #9
    I don't get it, I'm getting it wrong. a previous question asked for the change in potential energy which I got to be 107.623. So I think I'm using the wrong sign or something...

    I tried -178.223 + 107.623 and got -70.61N. I tried it with and without the negative sign and still got it wrong.
     
  11. Nov 17, 2007 #10

    Doc Al

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    That's the work done by friction, in Joules not Newtons. Now find the friction force.
     
  12. Nov 17, 2007 #11
    man..I suck at this physics stuff lol. I forgot that I found the W and not the force. Thx for your help though!!
     
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