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Homework Help: Find mechanical energy lost when a cyclist goes down a hill

  1. Oct 18, 2012 #1
    A cyclist has a speed of 5 m/s when she beings to coast down a hill, as shown in the figure. At the bottom of the hill, her speed has increased to 12m/s. the mass of the cyclist plus bicycle is 75kg.

    a) Calculate the work done by gravity on the cyclist and bicycle.

    b) Calculate the mechanical energy lost because of air resistance and friction. Explain.

    I found the work done(part a):

    Work = ΔK = Kf - Ki

    Work = [(1/2)mvf2 - (1/2)mvi2]

    Work = [(1/2)(75kg)(12m/s)2 - (1/2)(75kg)(5m/s)2]

    Work = (5400 - 937.5)Nm = J

    Work = 4463 J

    What I dont understand is how to find the air resistance and friction..i just need an idea of where to start


    Attached Files:

  2. jcsd
  3. Oct 18, 2012 #2
    What is the calculated work used for?

    What speed would cyclist have when there would be no need to do work?
  4. Oct 19, 2012 #3
    Gravity? I'm not sure what your asking..
  5. Oct 19, 2012 #4
    Let's say there would be no need to do any work. In that way, you would only have exchange between kinetic and potential energy. On top of the hill, your PE would be max and your KE would be min. On bottom of the hill, your KE would be max and your PE would be min. But such exchange is not possible because you have some losses and you need to do some work. Now I see that you neglected the potential energy.
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