Coefficient of friction

The coefficient of friction, also called coefficient of friction (symbol μ or f, dimensionless), is a measure of the frictional force in relation to the contact force between two bodies. The term belongs to a field of tribology.

  • 6.1 " μ is always smaller than one"
  • 6.2 " of static friction is static friction coefficient times the normal force "

Physical meaning

When specifying a coefficient of friction is made between sliding friction and stiction: When sliding friction, the friction surfaces move relative to each other while they do not in the static friction. In the case of the Coulomb friction is constant Gleitbeiwert. In practice a suitable temperature, velocity and pressure dependence can be seen, which indicates an influence of the surface modification and texture never perfectly planar surface ( but not to the coefficient of friction itself) and thereby affect the material properties in appearance.

Measured coefficient of friction of metals on polished surfaces to largely exclude a mechanical interlock (positive locking ) can. Always observe the adhesive and cohesive forces between the materials. There are formed depending on the material Van der Waals forces, or in polarized materials hydrogen- like forces between the surfaces. The best-performing material liability for ionic materials such as saline.

Calculation of the frictional force

Using the coefficient of friction allows the maximum prison or the sliding friction force between two bodies calculated.

In this case, the coefficient of friction and the normal force ( force perpendicular to the surface ), the friction force, respectively. The coefficient of friction therefore determines how large is the friction force in relation to the normal force; a higher coefficient of friction means more friction force.

For example, to push a metal block, you first need to apply a force that is greater than the static friction force. If the block then slides across the ground, it is sufficient to smaller sliding friction. Because the coefficient of friction from the ground ( dry, wet, ... ) depend, depend to the same extent also the friction forces on it.

In order to change the adhesion, one can also change the normal force, which in turn can be seen from the formula. At the level corresponding to the normal force of the weight. In motorsports, the normal force is increased by spoilers who use the coming of the front wind to press the vehicle to the ground. To racetracks curves are often tapered to match the adhesive surface of the resultant force of weight and centrifugal force; Thus, here, the normal force will be increased to obtain a higher adhesion.

Examples

The coefficient of friction tables are only approximate. The friction depends on many different factors ( material combination, surface, lubrication, temperature, humidity, wear and tear, normal force, etc. ), so that a table with the "right" values ​​can not be found.

The most accurate results are obtained from a test under real conditions. Again, however, be noted that the ratios between experimental and real commitment to change.

It always applies:

Static friction numbers

Maximum adhesion

A driven or braked tire opposite the surface on which it rolls, getting a slip. This slippage is so small that it can be ignored for many applications in small tangential forces transmitted. At higher tangential slip takes first weak, then increasingly to. This means that at a given contact pressure, a maximum tangential force can be transmitted. This is similar to the transition from static friction to sliding. The coefficient between the tangential force and the normal force is called adhesion. Its maximum indicates which force a tire can transfer a maximum as driving or braking force at a given normal force.

Coefficients of friction

Geometric interpretation

It can also be regarded as the tangent of the friction angle. This is the smallest angle at which a body would slip on an inclined plane downwards. It is true.

Example Auto: The tangent is part of everyday life as the slope of rising roads and slopes, which is indicated on road signs ( for example: mean gradient of 12%, over a length of 100 m increases the distance around 12 m). With a coefficient of static friction of one so you can climb slopes of up to 100% ( 45 °). Real gradeability of vehicles is limited mostly by the installed engine power and the total gear ratio of the transmission - exceptions are poor road conditions. When driving on ice or snowy road, the coefficient of static friction is very small, so that even slight slopes can not be overcome, or braking downhill no longer is possible.

Friction cone: Inside the friction cone (pictured right) are systems are also under stress conditions ( eg circuit to ground ) and referred to as self-locking, outside the friction cone reaches the frictional force is no longer sufficient to keep the system at rest, it occurs a movement. Relevant technical systems are, for example, worm gear, which are self-locking screws, depending on slope, material pairing and lubrication conditions or not.

Confines

Reaching the tensions caused by the forces yield stress ends, the scope of the Coulomb model. In its place the Reibfaktormodell.

Common errors

" Μ is always smaller than one"

It is sometimes said that should apply. simply means that the normal and frictional force are equal. For a number of material combinations, for example, silicone rubber or acrylic rubber -coated surfaces, the coefficient of friction is much greater than one.

" Static friction is static friction coefficient times the normal force "

Frequently, for the static friction formula

Specified. However, the thus calculated value is referred to only in the limit of the maximum thrust or pulling force in the opposite effect of the friction force and not the rest of the object possible. If this is exceeded, effective immediately, the smaller sliding friction:

This is evidently eg avalanches or landslides. Here are the crowds near the adhesive force. Small shocks can exceed the static friction locally.

Swell

268510
de