What is Coefficient of Friction?

An article by Yarina Gao    No Comments

Coefficient of Friction (COF) is the measurement of resistance to friction as related to the effect of how smooth or rough a surface is to prevent material to “slip” across the surface.

A low coefficient of friction for a pair of surfaces means they can move easily over each other. Ice and metal have a low coefficient of friction, so metal ice skates slide easily on ice.

A high coefficient of friction for a pair of surfaces means they cannot move easily over each other. Rubber tires and dry road surfaces have a high coefficient of friction, so they tend to grip each other. That means tires don’t slide easily when you apply the brakes or turn your vehicle into a corner.

Basically, there are two kinds of Coefficient of Friction: Static Coefficient of Friction and Kinetic Coefficient of Friction

Static Coefficient of Friction – also known as Coefficient of starting friction.

The static friction coefficient (μ) between two solid surfaces is defined as the ratio of the tangential force (F) required to produce sliding divided by the normal force between the surfaces (N)

μ = F /N

For a horizontal surface the horizontal force (F) to move a solid resting on a flat surface

F= μ x mass of solid x g.

If a body rests on an incline plane the body is prevented from sliding down because of the frictional resistance.   If the angle of the plane is increased there will be an angle at which the body begins to slide down the plane.  This is the angle of repose and the tangent of this angle is the same as the coefficient of friction.

It is that threshold of motion which is characterized by the Coefficient of static friction. The coefficient of static friction is typically larger than the coefficient of Kinetic Friction.

Kinetic Coefficient of Friction, also known as sliding Coefficient of Friction – Coefficient of friction after motion between surfaces is established

When the tangential force F overcomes the frictional force between two surfaces then the surfaces begins to slide relative to each other.   In the case of a body resting on a flat surface the body starts to move.    The sliding frictional resistance is normally different to the static frictional resistance.    The coefficient of sliding friction is expressed using the same formula as the static coefficient and is generally lower than the static coefficient of friction..

The kinetic coefficient of friction is calculated by:
μk = Ak / B  (where Ak is the reading on the gage after motion between the block and plane is established and B is the weight of the block.)

What kind of Factors affecting the friction between surfaces and How Friction affecting the products industries? We will discuss it later in future.