Physical Models of Friction
Friction effects can be imagined as resulting from two mountainous surfaces, one inverted above the other, as illustrated in Figure 1. The motion of one surface relative to the other causes the friction force. The height and sharpness of the mountains (asperities) represent the roughness of the surfaces. The lower mountains support the normal force (load) pressing down on the upper surface. The larger the load, the more deeply the upper surface penetrates into the lower, making it more difficult for the surfaces to move apart. This effect helps explain why the friction force depends on the load .