Friction

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

The frictional force is a function of the force pressing the surfaces together and the friction coefficient of the material interface. The friction coefficient (also known as the frictional coefficient or the coefficient of friction) is a scalar value used to measure the force of friction between two bodies. In particular:

Where F_f is the force of friction, F_pis the force perpendicular to the contact surface, and μ_f is the coefficent of friction. As is always true for friction, the force is exerted in a direction that opposes movement.

The coefficient of friction depends on the materials used - for example, ice on metal has a very low coefficent of friction (they rub together very easily), while rubber on pavement has a very high coefficent of friction (they do not rub together easily).

Types of friction

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

Static friction occurs when the two objects are not moving relative to each other (like a desk on the ground). This is typically denoted as . 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). This is typically denoted as , and is usually less than the static friction.

Rolling friction occurs when the two objects are moving relative to each other and one "rolls" on the other (like a shopping cart on the ground). This is typically denoted as .

The coefficent 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.

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 no longer hold the specified tolerances.

The energy used to overcome friction turns into heat. The work of movement will translate into deformation and heat that in the long run may affect the material's specification and the friction coefficient itself. Friction can cause solid materials to melt.

Friction may occur between solids, gases and fluids or any combination thereof. See aerodynamics and hydrodynamics. Also, friction occurs inside deforming objects.

A common way to reduce friction is by using a lubricant that is placed between the two surfaces, often radically lessening the percentage of the work that is turned into heat, which is the coefficient of friction. The science of friction and lubrication is called tribology. There is a new sort of quantum effect, called superlubricity. That means that there is no friction between two sliding objects. It has been measured in graphite

Lubricants to overcome friction need not always be thin, turbulent fluids; acoustic lubrication occurs when sound (measureable 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.