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Energy - Ski sliding down a ramp

  1. Nov 18, 2014 #1
    1. The problem statement, all variables and given/known data
    A ski starts from rest and slides down a 28 incline 65m long. If the coefficient of friction is 0.090, what is the ski's speed at the base of the incline? If the snow is level at the foot of the incline and has the same coefficient of friction, how far will the ski travel along the level?

    2. Relevant equations
    KE = (1/2)mv2
    PEg = mgh

    Note that I want to solve this problem using energy relations, not kinematics.

    3. The attempt at a solution
    Taking Point A to be the top of the ramp, point B to be right before the bottom.

    PEg = KE + Ediss
    mgh = (1/2)mv2 + Ff
    mgh = (1/2)mv2 + μFN
    mgh = (1/2)mv2 + μmg(sin 30)
    gh = (1/2)v2 + μg(sin 30)
    (9.8m/s2)(30.5157m) = (1/2)(v2) + (0.090)(9.8m/s2)(sin 30)

    v = 24.4 m/s.

    That answer was incorrect. I have not yet attempted the second question since I don't have a correct answer for the first one.
     
  2. jcsd
  3. Nov 18, 2014 #2
    First thing, The angle of incline is 28 ,not 30. Secondly work done by friction is not μN.
     
    Last edited: Nov 18, 2014
  4. Nov 18, 2014 #3

    ehild

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    The dissipated energy is not equal to the force of friction.
     
  5. Nov 18, 2014 #4
    Thanks for the reminder - although when I plugged in the new value, the answer changed very minimally...

    What should I set E diss equal to then??
     
  6. Nov 18, 2014 #5
    Energy dissipated is equal to the work done by friction . How do you calculate the work done by a force ?
     
  7. Nov 18, 2014 #6

    ehild

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    Do you think that energy is the same as force?????????
     
  8. Nov 18, 2014 #7
    Ff * d = W

    I think that will yield me the right answer. I would use the length of the incline, since that is the direction Friction was acting in. Then I'll plug it in. I'm right about to go to bed, so I'll do that in the morning.

    No I forgot to multiply it by Distance................!!! i get it now Silly Ritzycat
     
  9. Nov 18, 2014 #8

    ehild

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    Such things do happen :)
     
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