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Work due to static friction?

  1. Apr 16, 2012 #1
    1. The problem statement, all variables and given/known data
    A car is going down an incline with a constant acceleration.
    How much work was done on the car by the force of the road going down the hill? (neglect energy losses due to air resistance, rolling friction, etc.)

    3. The attempt at a solution
    There are 2 forces acting on the car from the road, the normal force and static friction. But the displacement of both the normal force and static friction are both 0, so how is static friction doing work?
     
  2. jcsd
  3. Apr 16, 2012 #2

    Doc Al

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    Staff: Mentor

    Technically, the static friction does no work on the car.
     
  4. Apr 16, 2012 #3
    That's exactly what I put, 0, and it was marked wrong. I'm trying to figure out why it was marked wrong
     
  5. Apr 16, 2012 #4
    Work done, ΔW= F.Δx
    Because Ff opposes the motion, it is directed opposite to x

    the work done on the car by friction,
    W=-Ff.x
     
    Last edited: Apr 16, 2012
  6. Apr 17, 2012 #5

    Doc Al

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    Staff: Mentor

    They probably wanted you to calculate F*Δx, where Δx is the displacement of the center of mass, not the point of contact. Strictly speaking, that's not work, but pseudowork.
     
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