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Question about pennies

  1. Sep 20, 2006 #1
    Let's say you have a penny placed above a table at some angle. (I don't think it matters much whether the bottom of the penny is touching the table.) You let go, and you hear the penny bounce back and forth, quickly coming to a stop. However, if you do the same thing while trying to spin the penny, you hear it vibrate faster and softer, but it takes longer to decay to 0. Why does it take longer to decay if the penny's spinning?
  2. jcsd
  3. Sep 20, 2006 #2


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    Well, a spinning penny has more energy, in the form of rotational kinetic energy, than one which is simply standing on it's rim, motionless, which simply has more gravitational potential energy than a penny lying flat on the table surface. The spinning penny also has angular momentum, which provides a torque, which resists the force of gravity.

    See some examples of rotational kinetics here -
    particularly http://hyperphysics.phy-astr.gsu.edu/hbase/top.html
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