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.

since you're a real machine, your ama is always less than your ima, because you can never totally eliminate friction.

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