Problem :

The coefficient of static friction, μs, between a given block of weight 50N and a surface has a value of .75. A horizontal force is applied to the block. How much force must be applied for the block to move?

When exactly enough force is applied for the block to move, the static frictional force will be equal to its maximum possible value. Thus:

Fs = Fsmax = μsFN

Because the block rests on a horizontal plane, the normal force is simply equal to the weight of the block: FN = 50N. Thus Fs = (50)(.75) = 37.5N. Recall that the static frictional force always cancels exactly with the force applied to the block. Thus the minimum necessary force to move the block is simply 37.5N.

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