Friction . In mechanics , or friction force to the resistance that opposes the movement ( kinetic friction force ) or the tendency to move ( static friction force ) of two surfaces in contact. It is generated due to imperfections, especially microscopic, between the surfaces in contact. These imperfections make the force between both surfaces not perfectly perpendicular to them, but forms an angle (the angle of friction) with the normal one. So this force The resultant is made up of the normal force (perpendicular to the contact surfaces) and the friction force, parallel to the contact surfaces.


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  • 1 Introduction
  • 2 Explanation of the origin of contact friction
  • 3 Laws of friction for solid bodies
  • 4 Mathematical formulation
  • 5 Static friction
  • 6 Values ​​of coefficients of friction
  • 7 Sources


Historically, the study of friction begins with Leonardo da Vinci who deduced the laws that govern the movement of a rectangular block that slides on a flat surface . However, this study went unnoticed.

In the 17th century , Guillaume Amontons , a French physicist, rediscovered the laws of friction by studying the dry slip of two flat surfaces. Amontons’ conclusions are essentially the ones we study in the General Physics books :

  • The friction force opposes the movement of a block that slides on a plane.
  • The friction force is proportional to the normal force exerted by the plane on the block.
  • The friction force does not depend on the apparent contact area.

French scientist Coulomb added one more property:

Once the movement has started, the friction force is independent of the speed.

Explanation of the origin of contact friction

Most surfaces, even those considered polished, are extremely rough on a microscopic scale. The peaks of the two contacting surfaces determine the actual contact area which is a small proportion of the apparent contact area (the area of ​​the base of the block). The area actual contact increases with increasing pressure (normal force) and the peaks are deformed.

The metals tend to cold welding, because the attractive forces that bind the molecules of a surface with other molecules. These welds have to be broken for slippage to occur. In addition, there is always the inlay of the peaks with the valleys . This is the origin of static friction .

When the block slides on the plane, cold welds are constantly broken and remade. But the amount of welds at any time is reduced below the static value, so that the kinetic friction coefficient is less than the static friction coefficient .

Finally, the presence of oil or grease on the surfaces in contact prevents welding by coating them with an inert material.

Friction laws for solid bodies

  • The friction force is of the same direction and sense opposite to the movement of the body.
  • The friction force is practically independent of the area of ​​the contact surface.
  • The friction force depends on the nature of the bodies in contact, as well as on the state in which their surfaces are.
  • The friction force is directly proportional to the normal force acting between the contact surfaces.
  • For the same pair of bodies, the friction is greater at the moment of starting than when the movement begins.
  • The friction force is practically independent of the speed with which one body moves over another.

Mathematical formulation

There are two types of friction: static and kinetic or dynamic . The first is the one that prevents an object from starting a movement and is equal to the net force applied to the body , only with the opposite sense (since it prevents movement). The second is a force of constant magnitude that opposes the movement once it has already started. In short, what differentiates one friction from the other is that the static one acts when the body is still and the dynamic one when it is in motion.

The static friction is always greater than or equal to the friction coefficient between the two objects (number that is measured experimentally and is tabulated) multiplied by the normal force. The dynamic friction, on the other hand, is equal to the friction coefficient, denoted by the Greek letter μ, by the normal one at all times. You don’t have a perfectly clear idea of ​​the difference between dynamic and static friction, but you tend to think that static is greater than dynamic, because when both surfaces remain at rest, ionic bonds may appear , or even micro-welds between The surfaces. This phenomenon is all the greater the more perfect the surfaces are. A more or less common case is that of seizure of a motor due to being stopped for a long time (it is not only seized by a very high temperature), since when the surfaces of the piston and the liner remain in contact and at rest for a long time, they can weld together.

Static friction

In the case of static friction, there is a range of forces that can be applied to the body and not a single one, as in the case of dynamic friction . For any force that meets the expression:

Static Friction

The body will remain at rest.


is the static friction coefficient.

N is the normal force between both surfaces.

Friction coefficient values

Friction coefficients of some substances
Materials in contact Static friction Kinetic friction
Ice // Ice 0.1 0.11
Glass // Glass 0.9 0.4
Glass // Wood 0.25 0.2
Wood // Leather 0.4 0.3
Wood // Stone 0.7 0.3
Wood // Wood 0.4 0.3
Steel // Steel 0.74 0.57
Steel // Ice 0.03 0.02
Steel // Brass 0.5 0.4
Steel // Teflon 0.04 0.04
Teflon // Teflon 0.04 0.04
Rubber // Cement (dry) 1.0 0.8
Rubber // Cement (wet) 0.3 0.25
Copper // Iron (cast) 1.1 0.3
Ski (waxed forest) // Snow (0ºC) 0.1 0.05
Human joints 0.01 0.003


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