Friction is the force that resists the movement of one material over another. It occurs when objects rub against each other. Friction is a force that is beneficial for many everyday activities, such as walking without slipping, the operation of brakes for vehicles, or writing on a chalkboard.
Some of the harmful effects of friction include the wearing out of shoes and clothing because of rubbing together, overheating of machine parts because of too much friction or increased work because extra force is needed to overcome friction.
It is often possible to increase or decrease the amount of friction between two surfaces. Friction between two surfaces can be reduced by making the surfaces smoother or by adding a lubricant such as oil, grease, soap or wax between the surfaces.