Penny slipping on an accelerating record

[alexito01]

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?

 

[arpanroy]

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