friction

In physics, the force that opposes the movement of two bodies in contact as they move relative to each other. The coefficient of friction is the ratio of the force required to achieve this relative motion to the force pressing the two bodies together.

Two materials with rough surfaces rubbing together will change kinetic energy into heat and sound energy. Friction is greatly reduced by the use of lubricants such as oil, grease, and graphite. A layer of lubricant between two materials reduces the contact, allowing them to slide over each other smoothly. For example, engine oil used in cars reduces friction between metal parts as they move against each other. Air bearings are now used to minimize friction in high-speed rotational machinery. In joints in the human body, such as the knee, synovial fluid plays a key role as a lubricant. In other instances friction is deliberately increased by making the surfaces rough - for example, brake linings, driving belts, soles of shoes, and tyres.

Friction is also used to generate static electric charges on different materials by rubbing the materials together.