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The key aspect of a force due to friction is that it has a maximum value. If a force is applied to a body that can move, and this body is resting of a fixed surface, the maximum force that can be applied to the body is a “coefficient of static friction” times the normal force between the body and the fixed surface. If this maximum value is exceeded, the body will begin to move. The force required to keep it moving is less than the force it took to get it moving. The magnitude of this force is a “coefficient of dynamic friction” times the normal force between the body and the surface. Both of these coefficients are found experimentally.
The most common mistake in solving problems involving friction, especially when the body has yet to move, is substituting the maximum value everywhere the friction force appears. The next most common mistake is when a friction force is found from an equilibrium process, and this force is greater than the maximum value. Keeping these two limitations in mind results in success dealing with friction. – Dr. Tom
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