Magnitude of Static Friction The magnitude of static friction f, is f& SHEN, where 14, is the coefficient of static friction and N is the magnitude of the normal force. The symbol ? means less than or equal to, implying that static friction can have a maximum value of pĂĄ& N. Static friction is a responsive force that increases to be equal and opposite to whatever force is exerted, up to its maximum limit. Once the applied force exceeds f.(max), the object moves. Thus, fs (max) = p&N. Magnitude of Kinetic Friction The magnitude of kinetic friction fg is given by where /4x is the coefficient of kinetic friction. A system in which f = 14% N is described as a system in which friction behaves simply. The transition from static friction to kinetic friction is illustrated in the figure below. fs, max N Ă‘ Motion fs = F F f 0 - F 0 |+- Static region -|- Kinetic region ~ W w (a) (b) (c) Impending motion Object moves (a) The force of friction f between the block and the rough surface opposes the direction of the applied force F. The magnitude of the static friction balances that of the applied force. This is shown in the left side of the graph in (c). (b) At some point, the magnitude of the applied force is greater than the force of kinetic friction, and the block moves to the right. This is shown in the right side of the graph. (c) The graph of the frictional force versus the applied force; note that fs (max) > fx. This means that Is > Hk. As you can see in the table below, the coefficients of kinetic friction are less than their static counterparts. The approximate values of p are stated to only one or two digits to indicate the approximate description of friction given by the preceding two equations.