Definition of friction.
Friction is a force that resists relative
motion between two surfaces in contact. Depending on the
application, friction may be desirable or undesirable.
Certain applications, such as tire traction on pavement and
braking, or when feet are firmly planted to move a heavy
object, rely on the beneficial effects of friction for their
effectiveness. In other applications, such as operation of
engines or equipment with bearings and gears, friction is
undesirable because it causes wear and generates heat, which
frequently lead to premature failure.
From a perspective of engineering, the
energy expended in overcoming friction is dispersed as heat
and is considered to be wasted because useful work is not
accomplished. This waste heat is a major cause of excessive
wear and premature failure of equipment. Two general cases
of friction occur: sliding friction and rolling friction.