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Homework Help: Skier problem

  1. Apr 11, 2010 #1
    1. The problem statement, all variables and given/known data
    if a skier goes down a hill that is 20m high and 70 m long perpendicular to the height... and she weighs 50kg and her intial veloctiy is 5 m/s. how fast is she going at the bottom if the force of gravity up the hill is 40N??
    I just dont get it!!!
    PLease help me


    2. Relevant equations
    total energy= kinetic energy + gravitational energy

    work = Fd



    3. The attempt at a solution
    Et= .5x50x(5x5)+ 50 x 9.8 x20
    = 10425

    and i dont know where to go from here please help me!!!!
    i have a test tomorrow and i dont understand
     
  2. jcsd
  3. Apr 11, 2010 #2

    tiny-tim

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    Welcome to PF!

    Hi peyton! Welcome to PF! :smile:
    What a terrible question :frown:

    how can gravity be up the hill … doesn't your professor know what "up" means??

    and how can anyone have a weight in kg? weight is in newtons, mass is in kg :rolleyes:

    hmm … if her mass is 50kg, the force of gravity down the hill would be about 140 N, not 40 N (it isn't, is it?) …

    i've no idea what this question means …

    if you have to answer it, I suggest you ignore the 40 N, which is indeed what you've already done …
    … now what speed does that correspond to? :smile:
    "total energy" isn't really a helpful concept (and nobody calls it "Et" :wink:) …

    I suggest you say KE + PE = constant, or ∆(KE) = -∆(PE)
     
  4. Apr 11, 2010 #3
    btw i am canadian
    the 10425J
    and it was the force of friction was 40N up hill

    i need to find the velocity at the bottom of the hill
     
  5. Apr 11, 2010 #4

    tiny-tim

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    D'oh! :rolleyes:

    In that case, you need to find the work done by the friction force, and use the work-energy theorem.
    ahh! :redface:
     
  6. Apr 11, 2010 #5
    then what ?
    find the difference then use that force to find the velocity at the bottom of the slope
     
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