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3. Dynamics eLearn.Punjab

3.3 FRICTION

Have you noticed why a moving ball stops? Why bicycle stops
when the cyclist stops pedaling?
Naturally there must be some force that stops moving objects.
Since a force not only moves an object but also stops moving
object.

The force that opposes the motion of moving objects is
called friction.

Figure 3.15: A cyclist keeps on pedaling to
overcome friction.

Friction is a force that comes into action as soon as a body is pushed or pulled over a surface. In
case of solids, the force of friction between two bodies depends upon many factors such as nature
of the two surfaces in contact and the pressing force between them. Rub your palm over different
surfaces such as table, carpet, polished marble surface, brick, etc. You will find smoother is the
surface, easier it is to move over the surface. Moreover, harder you press your palm over the
surface, more difficult would it be to move.

Why friction opposes motion?
No surface is perfectly smooth. A
surface that appears smooth has
pits and bumps that can be seen
under a microscope. Figure 3.17
shows two wooden blocks with
their polished surfaces in contact.
A magnified view of two smooth
surfaces in contact shows the gaps
and contacts between them.

Figure 3.17: A magnified view of the two surfaces in contact.

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