A rectangular block of wood placed on a flat surface has a string and spring balance attached to it so that a horizontal force can be applied (Fig. 2.4).
If a gradually increasing force is applied to the block it will, at first, continue to remain at rest since an equally increasing but oppositely directed force of friction F, comes into action at the under surface of the block. At any particular moment, we say that the pull P and the opposing frictional force F are in equilibrium. If we continue to increase the pull P, a stage will be reached when the block just begins to slip. At this point, the friction brought into play has reached its maximum value for the two surfaces concerned, and this is called the static friction.