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When a body rolls on a surface, the force
resisting the motion is termed rolling friction or rolling
resistance. Experience shows that much less force is
required to roll an object than to slide or drag it. Because
force is required to initiate and maintain rolling motion,
there must be a definite but small amount of friction
involved. Unlike the coefficient of sliding friction, the
coefficient of rolling friction varies with conditions and
has a dimension expressed in units of length.
Ideally, a rolling sphere or cylinder will
make contact with a flat surface at a single point or along
a line (in the case of a cylinder). In reality, the area of
contact is slightly larger than a point or line due to
elastic deformation of either the rolling object or the flat
surface, or both. Much of the friction is attributed to
elastic hysteresis. A perfectly elastic object will spring
back immediately after relaxation of the deformation. In
reality, a small but definite amount of time is required to
restore the object to original shape. As a result, energy is
not entirely returned to the object or surface but is
retained and converted to heat. The source of this energy
is, in part, the rolling frictional force.
A certain amount of slippage (which is the
equivalent of sliding friction) occurs in rolling friction.
If the friction of an unhoused rolling object is measured,
slippage effects are minimal. However, in practical
applications such as a housed ball or roller bearing,
slippage occurs and contributes to rolling friction.
Neglecting slippage, rolling friction is very small compared
to sliding friction.
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