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Practice Now   Tennis is played on a variety of court surfaces:
grass, clay, hardcourt, and even carpet. Players know that
matches on different surfaces are distinctly different games.
The ball bounces differently, and shoes slip on some surfaces
but stick on others. These differences are a result of
friction.    Friction is the force that resists motion. In most
beginning physics classes, friction is ignored. Concepts can be
understood and calculations made assuming friction to be non-
existent. Whenever physics intersects with the real world,
however, friction must be taken into account. Friction exists
between two touching surfaces because even the smoothest looking
surface is quite rough on a microscopic scale.  Every surface
has microscopic bumps, lumps, and imperfections, emphasized as
in the image above. If we try to slide the top block over the
lower block, there will be numerous collisions as the bumps
impact each other. The forward motion cause