A 55 kg roller skater is at rest on a flat skating rink, a 198 N horizontal force is needed to set the skater in motion. However, after the skater is in motion a horizontal force of 175 N is necessary to overcome the friction of 52N to keep
the skater moving at a constant velocity. Find the coefficient of kinetic friction.

Explanation:

To get the person Moving you have to overcome the static (means not moving) friction coefficient.  U(static)

To get the person going at the same speed you have to overcome the kinetic friction coefficient. U(Kinetic)

Force to get him moving is 198 N.   Force = ma = U(static)Mg

combining the 2 equations you get 198N = U(static)* 55kg *9.8m/s^2   Solve for U(static)

Same equation to keep him moving except with the dynamic force and the dynamic U

175N=  U(kinetic)*55kg*9.8m/s^2  Solve (U dynamic)

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