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Friction
Article by: Vernon D. Barger, Department of Physics, University of Wisconsin-Madison, Madison, Wisconsin.
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Resistance to sliding, a property of the interface between two
solid bodies in contact. Many everyday activities like walking or
gripping objects are carried out through friction, and most people have
experienced the problems that arise when there is too little friction and
conditions are slippery. However, friction is a serious nuisance in devices
that move continuously, like electric motors or railroad trains, since it
constitutes a dissipation of energy, and a considerable proportion of
all the energy generated by humans is wasted in this way. Most of this
energy loss appears as heat, while a small proportion induces loss of
material from the sliding surfaces, and this eventually leads to further
waste, namely, to the wearing out of the whole mechanism. Almost all
the frictional energy appears as heat at the interface between the sliding
surfaces. This frictional heat was used by humans in prehistoric times
to light fires, and this use survives today in the striking of matches, with
chemical combustion initiated by the temperature rise produced by
sliding (Fig. 1). However, frictional heat usually is a nuisance, and sliding
surfaces must often be cooled to prevent heat damage. Friction is stud-
ied in the field of tribology — the science of interactive surfaces in relative
motion — which also studies lubrication and wear.
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Content
• Mechanism
• Friction coefficient values
• Frictional oscillations
• Lubrication
Key Concepts
• Friction is a force that opposes the sliding of two parts or objects in contact, resulting
in both energy loss as heat and material loss from the sliding surfaces.
• The friction coefficient, f, is a value describing the relationship between the force of
friction and the force that is normal to the surface of contact between the two sliding
parts or objects.
• It is a misconception that the main cause of friction is surface roughness, as friction
coefficients are largely independent of surface texture.
• If friction decreases when the sliding speed increases, then frictional vibrations,
or oscillations, may be produced.
• Engineers use lubricants, such as oils, to reduce the amount of friction and any
associated oscillations to acceptable levels.
Fig. 1: Ignition of a match.
(Credit: KConstantine/Shutterstock.com)
