When two
objects touch, a certain portion of their surface areas will be in contact with each other. 'Contact area' refers to the fraction of this area that consists of the
atoms of one object being in contact with the atoms of the other object. Because objects are
rarely perfectly flat, the actual contact area (on a
microscopic scale) is usually much less than the contact area apparent on a
macroscopic scale. Contact area may depend on the
normal force between the two objects due to
deformation.
Friction and Contact Area
It is an
empirical fact for many
materials that F = μN, where F is the
frictional force for
sliding friction, μ is the
coefficient of friction, and N is the
normal force. There isn't a simple derivation for sliding friction's independence from area.
Methods for Determining Contact Area
One way of determining the actual contact area is to determine it indirectly through a physical process that depends on contact area. For example, the
resistance of a wire is dependent on the cross-sectional area, so one may find the contact area of a metal by measuring the
current that flows through that area (through the surface of an
electrode to another electrode, for example.)
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