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Need Help With a Problem in Physics 30

  1. Feb 27, 2005 #1
    I can't get this problem and wanted to see if anyone could help me out:

    A toy truck has a velocity of 6.0 m/s when it begins to role freely up a ramp inclined at 30.0 degrees. They toy has a mass of 5.0 kg, and the frictional forces present are 4.0N. What distance does the truck travel before stopping?

    I keep on getting 3.5m but the correct answer is 3.2m. Anyone know how to solve this?
     
  2. jcsd
  3. Feb 27, 2005 #2

    dextercioby

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    How about using the theorem of variation of KE...?I don't know if you have,but i found this to be the most elegant way

    Daniel.
     
  4. Feb 27, 2005 #3
    Well I've never heard of that theorem before, but maybe I've used it though. What I tried to do to solve this problem is figure out total energy before (Et = Ek + Ep) and then solve for when Ek is 0 after the toy has rolled up the ramp. The thing that complicates this is there's friction involved(heat, F * d). So I'm not sure how I have to include friction into this problem in order to get the right answer.
     
  5. Feb 27, 2005 #4

    Doc Al

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    Set the initial energy (purely KE, if you measure PE from the bottom of the ramp) equal to the final energy (PE + the heat due to friction = PE + F*d):
    [tex]{KE}_i = {PE}_f + F*d[/tex]
     
  6. Feb 27, 2005 #5

    dextercioby

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    For the record the theorem (proven in an elementary version by G.W.Leibniz) states that the variation of KE for a mechanical system is equal to the total work done by external forces acting on the system.In your case,the forces are friction & gravity and the mechanical system is composed only of one particle/body.

    Daniel.
     
  7. Feb 27, 2005 #6
    I tried to do that, but got 3.5 as my answer, so I was off by a little bit. I'll show you how I did this problem in deatil, maybe you'll be able to catch any mistakes?

    Et_i = Ek_i
    Ek_i = 0.5 * 5 * 6^2
    Ek_i = 90

    Et_f = Ep_f + (F * d)
    90 = -5 * -9.81 * h + (4 * (sin30/h))
    h = 1.75 (approx.)

    sin30 = 1.75/x
    x = 3.5 (approx.)
     
  8. Feb 27, 2005 #7

    dextercioby

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    The work done by friction is negative as well.Like the one performed by gravity.The lenght is approximately
    [tex]x\sim \frac{180}{57}m\sim 3.2 m [/tex]

    Check your calculations again.

    Daniel.
     
  9. Feb 27, 2005 #8

    Doc Al

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    Redo this calculation. x = h/sin30; you have it reversed.
     
  10. Feb 27, 2005 #9
    I tried that and ended up getting 4.38, so that doesn't work either.
     
  11. Feb 27, 2005 #10
    where did you get the 180 over 57 from?
     
  12. Feb 28, 2005 #11

    Doc Al

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    Well, try it again. You are making an arithmetic mistake:
    90 = (5)(9.8)h + (4)[2h] = 57h
    => h = ?
     
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