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Friction and Work with an object sliding down a Ramp Problem!

  1. Sep 20, 2009 #1
    A 70 kg crate starting at rest slides down a rough ramp 4.0 meters long, inclined at 30 degrees above horizontal. At the bottom, it is moving at 5.0 m/s. what was the work done by friction on the crate???

    Answers=
    A. -500 Joules
    B. 500 Joules
    C. 460 Joules
    D. -460 Joules
    E. 0.0 Joules

    I got this answer wrong and i was wondering why... so here is my work. i got the height to be 2meters by using sin 30

    Mgh - Wf = 1/2MV^2

    70 * 9.81 * 2 - wf = .5 * 70 * 5.0^2
    875 - wf = 1373.4
    wf =1373.4-875
    wf = 500 joules

    so i got positive 500 joules. can any body tell me what the answer truly is and what i did wrong?? thank you alot!!
     
  2. jcsd
  3. Sep 20, 2009 #2

    tiny-tim

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    Last edited by a moderator: Apr 24, 2017
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