Friction is the force that opposes the relative motion, or tendency of such motion, of two surfaces
in contact. It is not, however, a fundamental force, as it originates from electromagnetic forces and
exchange forces between atoms. In situations where the surfaces in contact are moving relative to
each other, friction between the two objects converts kinetic energy into heat (atomic
vibrations).
Friction between solid objects and fluids (gases or liquids) is called fluid friction. See also
aerodynamics and hydrodynamics. Friction is an extremely important force: it propels automobiles
and other ground transport and holds nails, screws, and nuts, along with many other
uses.
Equations
The classical approximation of the force of friction known as Coulomb friction, named after
Charles-Augustin de Coulomb, is expressed as
where μ is the coefficient of friction, N is the force normal to the contact surface, and Ff is the
force exerted by friction. This force is exerted in the direction opposite the object’s
motion.
This law mathematically follows from the fact that contacting surfaces have atomically
close contacts only over an extremely small fraction of their overall surface area, and
this contact area is proportional to load, until saturation takes place when all area is
in atomic contact and no further increase of friction force takes place. This simple,
although incomplete, representation of friction is adequate for the analysis of many physical
systems.
Coefficient of friction
The coefficient of friction, also known as the frictional coefficient, is a dimensionless scalar value
which describes the ratio of the force of friction between two bodies and the force pressing them
together. The coefficient of friction depends on the materials used. For example, ice on
metal has a low coefficient of friction, while rubber on pavement has a high coefficient of
friction.
Coefficients of friction need not be less than 1. Under good conditions, a tire on concrete
may have a coefficient of friction of 1.7. Magnetically attractive surfaces can have very
large friction coefficients, and glued or welded surfaces can effectively have extremely
large resistance to sliding. Sliding (dynamic) friction and static friction are distinct
concepts.
For sliding friction, the force of friction does not vary with the apparent area of contact between
the two objects. When the surfaces are adhesive, Coulomb friction becomes a poor approximation;
in this case the frictional force may depend on the area of contact.
The force of friction is always exerted in a direction that opposes relative movement, for kinetic
friction, or potential relative movement, for static friction, between the two surfaces. For example,
the drive wheels of an accelerating CAR experience a frictional force pointing forward;
without that force the wheels would spin and the rubber would slide backward along
the pavement. Thus friction does not necessarily oppose the direction of motion of the
vehicle; it opposes the relative sliding, or tendency to slide, between the tire and the
road.
The coefficient of friction is an empirical measurement and must be measured experimentally.
Rougher surfaces tend to have higher values. Most dry materials in combination give friction
coefficient values from about 0.3 to 0.6. A value of 0 would mean there is no friction at all.
Rubber in contact with other surfaces can yield friction coefficients from about 1.0 to
2.0.
A system with interlocking features between surfaces may be indistinguishable from
friction if the features are small, such as grains on two sheets of sandpaper or even
molecule-sized features. The coefficient of friction, multiplied by the normal reaction force on the
object, gives the magnitude of the Coulomb-friction approximation to the force opposing
sliding.
Static friction
Static friction, informally known as stiction, occurs when the two objects are not moving relative
to each other. The coefficient of static friction is typically denoted by μs. The initial force required
to get an object moving is often dominated by static friction. The maximum static friction is
typically larger than the kinetic friction.
Rolling friction occurs when one object rolls on another. In the ideal rolling-without-slipping
picture, the patch of the tire in contact with the ground is instantaneously stationary relative to
the ground. The coefficient of rolling friction is sometimes denoted by μr.
Limiting friction is the maximum value of static friction, or the frictional force that acts when a
body is just on the verge of motion on a surface.
Kinetic friction
Kinetic, or dynamic, friction occurs when two objects are moving relative to each other and rub
together. The coefficient of kinetic friction is typically denoted by μk and is usually less than the
coefficient of static friction.
From a mathematical point of view, one may consider a coefficient of friction that depends on
sliding velocity and whose value at zero velocity, μs, is related to the limiting value of the kinetic
coefficient μk as the velocity tends to zero. Such models provide one way to connect static and
kinetic friction in contact problems.
Examples of kinetic friction include:
- Sliding friction, when two objects rub against each other. Moving a book flat across a
desk is an example.
- Fluid friction, the resistive interaction between a solid object and a liquid or gas
through which it moves. Air resistance on an airplane and water resistance on a
swimmer are examples.
References
This entry is a derivative of the friction article from Wikipedia, the Free Encyclopedia.
Authors of the original article include Anakata, Rracecarr, Fresheneesz, Shenme, and
Zetawoof.
Wikipedia friction article
