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Skating and friction

  1. Feb 6, 2015 #1
    1. The problem statement, all variables and given/known data
    Which of the following could help to increase average velocity of the left skate of the person:

    Decreasing the static friction coefficient between the skate and the ground.

    Increasing the static friction coefficient between the skate and the ground.

    Decreasing the kinetic friction coefficient between the skate and the ground.

    Increasing the kinetic friction coefficient between the skate and the ground.

    Increasing the distance between the lines (circa 5m)

    Decreasing the distance between the lines.


    2. Relevant equations


    3. The attempt at a solution

    I think increasing the static friction and decreasing the kinetic friction would help. Static friction works in the direction of motion, does it?
     
  2. jcsd
  3. Feb 6, 2015 #2
  4. Feb 6, 2015 #3

    haruspex

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    What lines? Have you left out some of the problem statement? I would have guessed the distance between the feet, but 5m stretches more than just the imagination.
    Friction, whether static or kinetic, opposes relative motion of the surfaces in contact. It will work in the direction of motion of the body (skater, walker, car, train...) if the part in contact with the ground is trying to go the other way.
     
  5. Feb 7, 2015 #4
    Oh it is my fault. It is a video, you know. I have drawn it.

    Yes, I know that friction opposes motion but if you push ice with a skate to gain speed, isn't the static friction opposing pushing but favouring gliding?
    The example of a dog running on ice was simpler. :)
     

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  6. Feb 7, 2015 #5

    haruspex

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    Yes, indeed, static friction on the skate you're pushing back on is vital. But in your first post you wrote that 'static friction works in the direction of motion', and that is not always true. When a car brakes it opposes the motion of the vehicle. What is true in both cases is that it opposes relative motion of the surfaces in contact. In the case of the skate, the skater is pushing backwards on the skate. By resisting that relative motion, the static friction pushes the skater forwards.
     
  7. Feb 7, 2015 #6
    Oh, yes, I admit, I was very imprecise. I got it. Many thanks :)
    In the case of a dog it is better to increase both kinds of friction: the static and the kinetic because there is no sliding involved.
     
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