Coefficient of Kinetic Friction

Friction is an all-pervading force in nature. When you look around, you will find it at work, everywhere around you. Because of it, car tires get a good grip on roads, it makes walking possible, causes rivers to carve out caverns.

Friction causes the wear and tear of mechanical parts. There are innumerable examples of this phenomenon at work, that changes the face of the world as we know it.

Friction is the macroscopic force that opposes relative motion between any two surfaces, at the point of contact. The surfaces might be a solid and a fluid, two fluids, or a solid-fluid interface. When the two surfaces in contact are solids, it's called 'Dry Friction', for reasons that are self-explanatory.

This discussion entirely focuses on dry friction between solid surfaces. It comes in two types, which are static and kinetic friction. The former is between two surfaces that are not in motion, whereas the latter occurs between two moving surfaces, which are in contact with each other.

An example of kinetic friction is what car tires experience, while running on roads. Another example is the friction between a knife being sharpened and the rolling sharpening surface.

This phenomenon is the cause of extensive wear and tear of moving surfaces in contact with each other and it is the nemesis of machine efficiency. The force of kinetic friction is exerted in exactly the opposite direction of motion between two surfaces.

Dry friction has been theoretically modeled in physics so that it can be quantified. This coefficient is a concept that emerges in its empirical theory. The perpendicular force between two surfaces in contact with each other is called the normal force.

The Coulomb theory of kinetic friction is based on the following equation:

F_f = μ_Kinetic F_N

where μ_Kinetic is the coefficient of kinetic friction, F_f is the frictional force and F_N is normal force between the two surfaces.

As one can deduce from the equation, kinetic friction coefficient is the ratio between the frictional force exerted between the two surfaces and the normal force between them. Being a ratio, it has no units.

A knowledge of the coefficient between two surfaces gives a rough idea of the frictional force between the two surfaces. It is measured experimentally by simulating experiments. The same surface might have different coefficients, when in contact with different surfaces.

From the equation, one can derive:

μ_Kinetic = F_f/F_N

If you have to calculate the coefficient, you need values of the two forces, in units of Newton or Dyne. To calculate the value, all you have to do is substitute those values into the formula and divide to get the answer.