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Homework Help: Mistake in solving for work in physics?

  1. Apr 7, 2008 #1
    1. The problem statement, all variables and given/known data
    a student wearing a frictionless in-line skates on a horizontal surface is pushed by a friend with a constant force of 45N. How far must the student be pushed, starting from rest, so that her final kinetic energy is 352J?


    2. Relevant equations



    3. The attempt at a solution
    KE=1/2MV^2
    352=1/2*(45/9.81)*V^2
    Vf=12.39 and Vi=0 since it starts from rest

    Wnet=1/2MVf^2-1/2MVi^2
    1/2*(45/9.81)*12.39^2=352.31

    Wnet=FnetDCosX
    352.31=45dcos0
    d=7.83m

    I just wanted to know if that was a correct way of solving it or did i make a mistake somewhere?
    thx for the help.
     
  2. jcsd
  3. Apr 7, 2008 #2

    Doc Al

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    Staff: Mentor

    Since you are given the final KE, these calculations are not needed. (Note how error creeps in: 352.31 should really be 352.)

    This is all you need. (Use the given value for KE.)
     
  4. Apr 7, 2008 #3
    the Wnet is always the same as KE?
     
  5. Apr 7, 2008 #4

    Doc Al

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    Staff: Mentor

    The net work will equal the change in KE. That's the so-called Work-KE theorem.
     
  6. Apr 7, 2008 #5
    ooo thxx hehe
     
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