1. The problem statement, all variables and given/known data A penny of mass 3.1g rests on a small 20.0g block supported by a spinning disk of radius(r) 12cm.If the coefficients of friction between block and disk are 0.75(static) and 0.64(kinetic) while those for the penny and block are 0.45(kinetic) and 0.52(static).What is the maximum rate of rotation (in revolutions per minute) that the disk can have before either the block or the penny starts to slip? 2. Relevant equations Fr=[tex]\mu[/tex]R F=m(omega)2r where omega=angular velocity 3. The attempt at a solution Here's the force diagram I drew, http://img24.imageshack.us/img24/8028/cicularmotion.png" [Broken] but I'm not very sure what I should do next. I guess I should use Fr or Fr'=[tex]\mu[/tex]R(or S as labelled in the diagram) but I don't know which [tex]\mu[/tex] to use,the coefficient of static friction or kinetic friction ? and what is the difference between the 2 types of coefficients? I know that static friction is the friction acting on the block when it is just about to move and is slightly more than the dynamic friction,but I'm not sure what to do with this info?