Force that resists sliding or rolling of one solid object over another. Some friction is beneficial, such as the traction used to walk without slipping.
Most friction, though, is undesirable opposition to motion, such as between moving parts of machines. For example, about 20% of the work done by an automobile engine is needed to overcome friction between moving parts. Friction is a result of attractive forces between the contact regions of two bodies, and the amount of friction is almost independent of the area of contact. Kinetic friction arises between surfaces in relative motion, static friction acts between surfaces at rest with respect to each other, and rolling friction occurs when an object rolls over a surface.
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