This article is about the resistive force. For the Japanese rock band, see Friction (band).

In physics, friction is the resistive force that occurs when two surfaces travel along each other when forced together. It causes physical deformation and heat buildup.

The frictional force is a function of the force pressing the surfaces together and the coefficient of friction between the materials. In particular:

F_f = F_pμ_f

where F_f is the force of friction, F_p is the force perpendicular to the contact surface (usually called the normal force), and μ_f is the coefficient of friction.

The coefficient of friction (also known as the frictional coefficient or the friction coefficient) is a scalar value used to calculate the force of friction between two bodies. The coefficient of friction depends on the materials used -- for example, ice on metal has a very low coefficient of friction (they rub together very easily), while rubber on pavement has a very high coefficient of friction (they do not rub together easily). It is interesting to note that, contrary to common belief, the force of friction is invariant to the size of the contact area between the two objects.

The force of friction is always exerted in a direction that opposes movement. For example, a chair sliding to the right across a floor experiences the force of friction in the left direction.

Types of friction

The coefficient of friction also depends on the type of friction. There are two general types of friction:

• Static friction 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 μ_s. The initial force to get an object moving is often dominated by static friction, sometimes called "stiction".
• Kinetic friction occurs when the 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 μ_k, and is usually less than the coefficient of static friction.
  • Rolling friction occurs when the two objects are moving relative to each other and one "rolls" on the other (like a car's wheels on the ground). The coefficient of rolling friction is typically denoted as μ_r.

The coefficient of friction is an empirical measurement -- it has to be measured experimentally, and cannot be calculated.

Since the total amount of friction depends on the path an object takes, friction is not a conservative force.

Energy Loss Associated With Friction

When an object is pushed along a surface with coefficient of friction μ_k and a perpendicular (normal) force acting on that object directed towards the surface of magnitude N, then the energy loss of the object is given by:

U = Nμ_kd

Where d is the distance travelled by the object whilst in contact with the surface. This equation is identical to Energy Loss = Force x Distance as the frictional force is a non-conservative force. Note, this equation only applies to kinetic friction, not rolling friction.

Physical results of friction

Physical deformation is associated with friction. While this can be beneficial, as in polishing, it is often a problem, as the materials are worn away, and may no longer hold the specified tolerances.

The work done by friction can translate into deformation and heat that in the long run may affect the surface's specification and the coefficient of friction itself. Friction can in some cases cause solid materials to melt.

Friction may occur between solids, gases and fluids or any combination thereof. See aeroscentics and hydroathletics.

A common way to reduce friction is by using a lubricant such as oil that is placed between the two surfaces, often dramatically lessening the coefficient of friction. The science of friction and lubrication is called tribology. Superlubricity, a recently-discovered effect, has been observed in graphite: it is the substantial decrease of friction between two sliding objects, approaching zero levels - a very small amount of frictional energy would be dissipated due to electronic and/or atomic vibrations.

Lubricants to overcome friction need not always be thin, turbulent fluids; acoustic lubrication occurs when sound (measurable in vacuum by placing a microphone on one element of the sliding system) permits vibration to introduce separation between the sliding faces. World War II Panzer tank treads lubricated by their own squeak provide the most famous, if serendipitous, example.

"Friction" is a track on Television's 1977 release Marquee Moon.