Friction and Surfaces
I am trying to recall my physics classes. Is friction
independent of surface area? If you have two 50 pound boxes with
different surface contact areas to the floor. Will both require the same
amount of force to start moving the boxes and keep them moving?
Yes, friction is independent of surface area. However, it is dependant on
the types of surfaces in contact. A wooden block will move at a lower force
on ice than it would on concrete. Also, friction values differ in static
friction and dynamic friction. That is, it takes more force to start a
block moving from rest (static friction) than it does to keep it moving
(dynamic friction). Most elementary physics books will have an entire
chapter on frictional forces.
Friction is usually idealized as a force independent of surface area
and proportional to the normal force pressing surfaces together. In
this idealization, the two boxes you describe would require the same
force to move.
From the experimental data that support this idealization, you could
equally well claim that the frictional force is proportional to the
surface area and to the normal force per unit area pressing the
Clearly these idealizations break down when the surface area becomes
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Update: June 2012