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Work done and friction

  1. Oct 31, 2007 #1
    If work done is defined as F X distance moved in the direction of this force, is friction doing work on a moving car since if the car is moving to the right friction is actually acting to the left? or should the definition of work done be F x distance moved parallel to the line of action of this force in which case the displacement and force can be in totally opposite directions.
     
  2. jcsd
  3. Oct 31, 2007 #2

    Doc Al

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    If the car moves to the right but friction acts to the left, then the work done will be negative. That would be the case when braking. If the car is accelerating, then the force will point in the direction of motion and the work done will be positive.
     
  4. Oct 31, 2007 #3
    Thank you so much, that's made it very clear.
     
  5. Dec 16, 2007 #4
    Consider a car moving forward to the right as you look at the car. This implies the the tires rotating in a clockwise direction. As a point on the tire comes around to the bottom where it contacts the road, the direction the tire is moving relative to the road is to the left. Friction, in this case, is then applied to the tire toward the right --> in the direction of the cars motion. As previously mentioned, friction acts opposite to the direction of motion. The "motion" being the motion of the tire.
     
  6. Dec 16, 2007 #5

    Doc Al

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    The direction of friction depends on whether the car is accelerating or deaccelerating. Friction acts to oppose slipping between the surfaces.
     
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