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To visualize sliding friction, imagine a
steel block lying on a steel table. Initially a force F
(action) is applied horizontally in an attempt to move the
block. If the applied force F is not high enough, the block
will not move because the friction between the block and
table resists movement. If the applied force is increased,
eventually it will be sufficient to overcome the frictional
resistance force f and the block will begin to move. At this
precise instant, the applied force F is equal to the
resisting friction force f and is referred to as the force
of friction.
In mathematical terms, the relation between
the normal load L (weight of the block) and the friction
force f is given by the coefficient of friction denoted by
the Greek symbol µ. Note that in the present context, “normal”
has a different connotation than commonly used. When
discussing friction problems, the normal load refers to a
load that is perpendicular to the contacting surfaces. For
the example used here, the normal load is equal to the
weight of the block because the block is resting on a
horizontal table. However, if the block were resting on an
inclined plane or ramp, the normal load would not equal the
weight of the block, but would depend on the angle of the
ramp. Since the intent here is to provide a means of
visualizing friction, the example has been simplified to
avoid confusing readers not familiar with statics.
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