# PlanetPhysics/Friction

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 the electromagnetic forces and exchange force between atoms. In situations where the surfaces in contact are moving relative to each other, the 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:
   $F_{f}=\mu N$
where Î¼ 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 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 thus 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 (they slide past each other easily), while rubber on pavement has a high coefficient of friction (they do not slide past each other easily). 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 together surfaces have infinite friction coefficient. Sliding (dynamic) friction and static friction are distinct concepts. For sliding friction, the force of friction does not vary with the area of contact between the two objects. This means that sliding friction does not depend on the size of the contact area.
When the surfaces are adhesive, Coulomb friction becomes a very poor approximation (for example, Scotch tape resists sliding even when there is no normal force, or a negative normal force). In this case, the frictional force may depend on the area of contact. Some drag racing tires are adhesive in this way (see, for example, [1]).
The force of friction is always exerted in a direction that opposes movement (for kinetic friction) or potential movement (for static friction) between the two surfaces. For example, a curling stone sliding along the ice experiences a static force slowing it down. For an example of potential movement, the drive wheels of an accelerating CAR experience a frictional force pointing forward; if they did not, the wheels would spin, and the rubber would slide backwards along the pavement. Note that it is not the direction of movement of the vehicle they oppose, it is the direction of (potential) sliding between tire and road.
The coefficient of friction is an empirical measurement -- it has to be measured experimentally, and cannot be found through calculations. Rougher surfaces tend to have higher values. Most dry materials in combination give friction coefficient values from 0.3 to 0.6. It is difficult to maintain values outside this range. A value of 0.0 would mean there is no friction at all. Rubber in contact with other surfaces can yield friction coefficients from 1.0 to 2.0. A system with "interlocking teeth" between surfaces may be indistinguishable from friction, if the "teeth" are small, such as the grains on two sheets of sandpaper or even molecule-sized "teeth".
The coefficient of friction, when multiplied by the reaction force on the object by the contact surface, will give the total frictional force opposing sliding on the object.

### Static friction

Static friction (informally known as stiction) occurs when the two objects are not moving relative to each other (like a desk on the ground). The coefficient of static friction is typically denoted as Î¼s. The initial force to get an object moving is often dominated by static friction. The static friction is in most cases higher than the kinetic friction. That is why you feel a jerk when starting to move and when stopping.
Rolling friction occurs when one object "rolls" on another (like a car's wheels on the ground). This is classified under static friction because the patch of the tire in contact with the ground, at any point while the tire spins, is stationary relative to the ground. The coefficient of rolling friction is typically denoted as Î¼r.
Limiting friction is the maximum value of static friction, or the force of friction 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 (like a sled on the ground). The coefficient of kinetic friction is typically denoted as Î¼k, and is usually less than the coefficient of static friction. From the mathematical point of view, however, the difference between static and kinematic friction is of minor importance: Let us have a coefficient of friction which depends on the sliding velocity and is such that its value at 0 (the static friction Î¼s ) is the limit of the kinetic friction Î¼k for the velocity tending to zero. Then a solution of the contact problem with such Coulomb friction solves also the problem with the original Î¼k and any static friction greater than that limit.
Examples of kinetic friction:
* Sliding friction is when two objects are rubbing against each other. Putting a book flat on a desk and moving it around is an example of sliding friction
* Fluid friction is the friction between a solid object as it moves through a liquid or a gas. The drag of air on an airplane or of water on a swimmer are two examples of fluid friction.