To explain the car breaks answer above so it makes sense -- when you apply the breaks, metal (or graphite, etc.) plates squeeze on a part of the wheel. There is friction involved, which is needed to slow and stop the wheel from turning. Without friction, it would just slip, and the wheel would never slow down. By the same token, over time that friction (and the heat caused by friction) will wear down the break pads, and they will need to be replaced. So -- this friction is both a good thing, and a bad thing!
An example is the braking system in an automobile, where the forward motion of the car is turned into heat, thus slowing the vehicle.
friction, resistance offered to the movement of one body past another body with which it is in contact. In certain situations friction is desired. Without friction the wheels of a locomotive could not “grip” the rails nor could power be transmitted by belts. On the other hand, in the moving parts of machines a minimum of friction is desired; an excess of friction produces heat, which in turn causes expansion, the locking of the moving parts, and a consequent breakdown of the machinery. Lubrication is important in minimizing friction as are also such devices as ball and roller bearings.
