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Homework Help: Please Check to See if I'm Doing Work-Energy Problem Correctly

  1. Jan 13, 2008 #1
    [SOLVED] Please Check to See if I'm Doing Work-Energy Problem Correctly

    1. The problem statement, all variables and given/known data

    A child riding a bicycle has a total mass of 40 kg. The child approaches the top of a hill that is 10 m high and 100 m long at 5 m/s. If the force of friction between the bicycle and the hill is 20 N, what is the child's velocity at the bottom of the hill?

    2. Relevant equations

    K = 1/2 * mv^2, U = mgh, W = delta E, W = Fx * cos(theta)

    3. The attempt at a solution

    Can someone check to see if I'm doing this correctly. I think what they're trying to say is that you're already at the top.

    I would first do K-initial + U-initial = 4420 J. Then Then set that to Fx and solve for F which is 44.2 N. Subtract 20 N from that and you get 24.2 N and multiply it back by x which gives you 242 J. Set that to K-final and solve for v and you get 11 m/s.

    Did I do everything correctly?
     
    Last edited: Jan 14, 2008
  2. jcsd
  3. Jan 14, 2008 #2

    Shooting Star

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    Homework Helper

    Use the energy equation properly. What you have written seems complicated to me. Where is cos theta comin from?

    KE at bottom = KE at top + PE at top + Work done against friction.
     
  4. Jan 14, 2008 #3

    Kurdt

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    I get 11m/s as well and I used the kinematic equations. So I guess you've done everything correctly.
     
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