FRICTION

a force which is brought into play when one body moves or tends to move over another. It always acts as a resistance to motion; when it prevents motion, it is called static friction; when slipping takes place, it is known as kinetic friction. Static friction may have any value between zero and a maximum which is reached just before slipping takes place, and which is called limiting friction. When slipping begins, the friction force suddenly diminishes to a smaller value; it is a matter of common experience that a greater force is required to start a ma in raotion against friction than is nece ary to keep the ma moving. Friction is proportional to the normal or perpendicular pre ure between the two surfaces in contact, and the ratio of the friction force to the normal pre ure is called the coefficient of friction. Friction is independent of the area of the rubbing surfaces, and when motion takes place, the friction is independent of the speed of slipping except for very low speeds. These laws a ume that no change takes place in the surfaces as the result of rabbing— a condition which is never realized. Friction may be diminished by smoothing the surfaces, by the use_ of rollers, wheels, or ball-bearings, and by applying lubricants such as oil, graphite, Friction is the ultimate force employed in stopping vehicles by means of brakes. It is by means of friction we are able to hold objects in our hands, and to walk on the ground. Friction always produces heat, and, when the surfaces are unlike, it also generates electricity. The particles of a fluid exerg friction against each other, and against any solids in contact with them.