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, such as a refrigerator or bookcase, along the ground. In other cases, friction is helpful. Friction between people's shoes and the ground allows people to walk by pushing off the ground without slipping. On a slick surface, such as ice, shoes slip and slide instead of gripping because of the lack of friction, making walking difficult. Friction allows car tires to grip and roll along the road without skidding. Friction between nails and beams prevents the nails from sliding out and keeps buildings standing.

When friction affects a moving object, it turns the object's kinetic energy, or energy of motion, into heat. People welcome the heat caused by friction when rubbing their hands together to stay warm. Frictional heat is not so welcome when it damages machine parts, such as car brakes. Friction occurs in part because rough surfaces tend to catch on one another as they slide past each other. Even surfaces that are apparently smooth can be rough at the microscopic level. They have many ridges and grooves. The ridges of each surface can get stuck in the grooves of the other, effectively creating a type of mechanical bond, or glue, between the surfaces.

Scientists do not yet fully understand the details of how friction works, but through experiments they have found a way to describe frictional forces in a wide variety of situations. The force of friction between an object and a surface is equal to a constant number times the force the object exerts directly on the surface. The constant number is called the coefficient of friction for the two materials and is abbreviated µ. The force the object exerts directly on the surface is called the normal force and is abbreviated N. Friction depends on this force because increasing the amount of force increases the amount of contact that the object has with the surface at the microscopic level. The force of friction between an object and a surface can be calculated from the following formula: F = µ × N In this equation, F is the force of friction, µ is the coefficient of friction between the object and the surface, and N is the normal force.