Friction
. Friction, force that opposes the motion of an object when the object is in contact with another object or surface. Friction results from two surfaces rubbing against each other or moving relative to one another. It can hinder the motion of an object or prevent an object from moving at all. The strength of frictional force depends on the nature of the surfaces that are in contact and the force pushing them together. This force is usually related to the weight of the object or objects. In cases involving fluid friction, the force depends upon the shape and speed of an object as it moves through air, water, or other fluid. Friction occurs to some degree in almost all situations involving physical objects. In many cases, such as in a running automobile engine, it hinders a process. For example, friction between the moving parts of an engine resists the engine’s motion and turns energy into heat, reducing the engine’s efficiency. Friction also makes it difficult to slide a heavy object, s