The Laws of Dry Friction. Coefficients of Friction

• Block of weight W placed on horizontal surface. Forces acting on block are its weight and reaction of surface N.
•  Small horizontal force P applied to block. For block to remain stationary, in equilibrium, a horizontal component F of the surface reaction is required. F is a static-friction force.
•  As P increases, the static-friction force F increases as well until it reaches a maximum value Fm.
  

•  Further increase in P causes the block to begin to move as F drops to a smaller kinetic-friction