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Gravitational potential energy.

  1. Nov 23, 2008 #1
    1. The problem statement, all variables and given/known data
    Hey guys, just wondering if you guys could help me out a bit.

    A boy on a sled with a total mass of 72kg slides from rest down a hill, which is 35m high. The friction force between sled and snow has a magnitude of 250N. If the slope of the hill makes an angle of 37 degrees with the horizontal, find:
    a) the gravitational potential energy of the sled at the top of the hill relative to the base.

    2. Relevant equations

    If i am not mistaken the equation for Gav.pot energy is
    U= GMe * m

    and the value of g can be found as g = G * Me/ (R + h)2

    3. The attempt at a solution

    I know i have the following from the question:

    m= 72kg
    Ff= 250N

    now what i dont get is where the 37 degrees is supposed to come in and i am not quite sure that that is the equation that i am supposed to use. Could someone please explain to me how i would begin this question? I want to solve it my slef but can someone point me in to a direction. thank you.

    edit: wait a second, i think i was doing it wrong. Would it be like E1g = mgh = (72kg)(9.8m/s2)(35m) = 24696 J ?
    Last edited: Nov 23, 2008
  2. jcsd
  3. Nov 23, 2008 #2

    Doc Al

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    Staff: Mentor

    Much simpler would be to use PE = mgh, which is perfectly good near the Earth's surface.

    Maybe you don't need it for part a. :wink:
  4. Nov 23, 2008 #3

    Doc Al

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    Staff: Mentor

    Now you've got it.
  5. Nov 23, 2008 #4

    Ok cool thanks guys, i think i can handle it form here ^_^. look like i will be studying all day for that physics test tomorrow :frown:
  6. Nov 23, 2008 #5
    Sorry for posting again but now i am stuck at part c of the question which is : the speed of the sled at the bottom of the hill.

    How would i find this out? I know i have to use the 37 degrees somehow but i am stuck, I now have the following variables.

    h1 = 35m
    Ff = 250N
    gav.pot energy : 24696J
    Work: 8750J

    Its been a while since i have had to find speed so a hint would be helpful =)
  7. Nov 23, 2008 #6

    Doc Al

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    Staff: Mentor

    What's the work done by friction? Hint: What's the distance traveled by the sled?

    Set up an energy equation.
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