Coefficient of Kinetic Friction

Friction is an all pervading force in nature. When you look around, you will find friction at work everywhere around you. It is because of friction that car tires can get a good grip on roads, it is friction that makes walking possible, it is friction that causes rivers to carve out caverns and it is friction that causes the wear and tear of mechanical parts. There are innumerable examples of frictional phenomena that change the face of the world as we know it.

What is Kinetic Friction?
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. There are two types of dry friction. They are static and kinetic friction. Static friction is friction between two surfaces that are not in motion, whereas kinetic friction is friction between two moving surfaces in contact with each other.

An example of kinetic friction is the friction experienced by car tires while running on roads. Another example is the friction between a knife being sharpened and the rolling sharpening surface. Kinetic friction is the cause of extensive wear and tear of moving surfaces in contact with each other and it is the nemesis of machine efficiency. Force of kinetic friction is exerted in exactly the opposite direction of motion between two surfaces.

What is Kinetic Friction Coefficient?
Dry friction has been theoretically modeled in physics, so that it can be quantified. This coefficient is a concept that emerges in the empirical theory of dry friction. 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 above, 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 of kinetic friction when in contact with different surfaces.

Calculation
From the equation above, one can derive:

μ_Kinetic = F_f / F_N

If you have to calculate this friction coefficient, you will be provided with values of two forces in units of Newtons or Dyne. To calculate the value, all you have to do is substitute those values into the above formula and divide to get the answer. The coefficient of static friction can also be calculated using the same equation of coulomb theory of dry friction. Friction can never be eliminated totally but its damaging effect in machinery can be reduced.