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Angular accelaration problem help

  1. Oct 15, 2008 #1
    A penny rests on a record at a radius r=0.200m. The record player is turned on and the record steadily accelerates with angular acceleration alpha=20.0 rad/s^2. The coefficient of static friction between the record and the penny is 0.500. At what time will the penny begin to slip?
  2. jcsd
  3. Oct 15, 2008 #2
    Basically the penny will begin to slip when the centrifugal force is greater than the frictional force.
    Your frictional force is Umg where U is the coefficient of friction(0.5) in this case.
    Ur centrifugal force will be mw^2r where w is the angular velocity.

    solve for Umg=mw^2r first to get w at which it slips...

    then use w=angular accel* t to get ur t
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