Consider a block at rest on a rough horizontal surface. A force P acts to the right on the block, with the magnitude of P being increased gradually. From what we have seen in lecture, the friction force f acting on the block as a result of the application of the force P will behave as follows:
- The direction of f will always oppose the motion/impending motion of the block. Since the block would move to the right in the absence of friction, f will always act to the left on the block.
- For low values of P, the block will remain at rest (equilibrium). From equilibrium considerations, we see that P = f while the block remains in equilbrium.
- The maximum friction force that can act on the block is given by fmax = muS*N, wheremuS is the coefficient of static friction. From these we see that the block will be in a state of impending slip when P = fmax = muS*N.
- As P is continually increased, the block will now slip, with the friction force acting on the block being: f = muK*N, where muK is the coefficient of kinetic friction, with f remaining constant thereafter as P is increased.
Animation
Consider the animation from a simulation of this problem corresponding to muS = 0.8 and muK =0.4. As described above, the friction force f matches the applied force P one-to-one up until slipping is initiated at f = fmax = muS*N. As P is increased further, the friction force f remains constant at f = muK*N.
Please note that f = muS*N only at the single value of P at which the block is in impending motion. Remember this!