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The Wheel

  1. Mar 21, 2014 #1
    Hello I know this sounds stupid but my friend told me about something that made me think. So what he told me was that every point on a car wheel is still for a very small fraction
    of time

    ImageUploadedByPhysics Forums1395443479.326283.jpg

    Point x is stationary at this point in time but as the wheel moves forward it revolves around x and stays at place y

    ImageUploadedByPhysics Forums1395443887.692809.jpg

    So it looks like this
    ImageUploadedByPhysics Forums1395443973.719997.jpg

    Does it do this or does it move with the rest of the wheel ?
  2. jcsd
  3. Mar 21, 2014 #2


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    "Still" is a concept involving motion. Motion is a relative phenomenon. EVERYTHING is still relative to its own frame of reference and anything in that frame of reference. When you a running down the street, you are standing still relative to your belt buckle.
  4. Mar 21, 2014 #3


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    The part of the wheel in contact with the ground is pushing the car forward. It is going slightly backward if the tire slips or stretches at all. At the top, the wheel is going forward. So somewhere in between, very near the bottom, it's forward velocity must be zero. You could reasonable say that it has zero velocity exactly at the bottom. But if you want to be really picky, its forward velocity may not be exactly at the bottom and it might have some downward velocity there. In other words, its forward velocity may not be zero at the exact same time that its downward velocity is zero.
  5. May 26, 2014 #4
    You could also say that that point is still relative to the road but not to the car
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