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Conservation of motion

  1. Oct 15, 2009 #1
    1. The problem statement, all variables and given/known data
    boy in a wheelchair (total mass, 48.7 kg) wins a race with a skateboarder. He has a speed of 1.23 m/s at the crest of a slope 2.37 m high and 14.0 m long. At the bottom of the slope, his speed is 6.27 m/s. If air resistance and rolling resistance can be modeled as a constant frictional force of 41.8 N, calculate the work he did in pushing forward on his wheels during the downhill ride.


    2. Relevant equations
    Ei-Ef=Wfriction+Woutside forces

    1/2mv^2, mgh
    3. The attempt at a solution
    cant seem to set this up right. dont know the equations for the 2 work
     
  2. jcsd
  3. Oct 15, 2009 #2
    Can't you think of a general expression for work that you could use here?
     
  4. Oct 15, 2009 #3
    Not of the outside forces
     
  5. Oct 15, 2009 #4
    that would be cool to get some help instead of pointing out that im not smart, thats why im asking for help
     
  6. Oct 16, 2009 #5
    What are the outside forces?

    In particular, what is the work done by gravity?
     
  7. Oct 16, 2009 #6
    MgH=(48.7)(9.8)(14)
     
  8. Oct 16, 2009 #7
    OK but H=2.37 m here, not 14m.

    Now you just need to calculate the work done by friction.
     
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