A coin is placed 12.0 cm from the axis of a rotating turntable of variable speed. When the speed of the turntable is slowly increased, the coin remains fixed on the turntable until a rate of 39.0 rpm (revolutions per minute) is reached, at which point the coin slides off.

fs = m / R

   = mR    ........... where w = 2f

then fs = µsN = µs×mg = R

µs = R/g , since w = 2f = 2× 39.0

                                            = 4.084

µs = 0.12× / 9.8

     =  0.204

Explanation:

when using this equation it is important to look at the units you working with so that it would be much easier to workout the solution.

the part of the wave he is travelling on is the crest