What is Friction?

What is Friction?


Friction is a force that occurs between two surfaces. It can be considered as an opposing force, working to prevent motion of one surface relative to the other.

 

How does friction work?

For many years it was thought that surface roughness was the only factor involved. Imagine trying to slide two pieces of sandpaper across each other; the abrasive nature of the papers causes a resistive force which can be felt. The ‘rougher’ the sandpaper, the more abrasive the reaction. Even the smoothest appearing materials will have hills and valleys in their surfaces. This ‘sandpaper effect’ is an important component of friction, but it is now known that another cause is important.

At the ‘microspcopic level’, there is a force of adhesion between surfaces; Molecular, or electro-magnetic, forces attract the molecules of the two material surfaces together, forming what is like an adhesive bond; this molecular adhesion needs to be broken for two surfaces to slide relative to each other. Different materials have different levels of ‘stickiness’ and we will review this later.

Another ‘component’ of friction is when surfaces deform or, more usually, the deformation of a softer surface by a harder one.

As can be seen, the ploughing effect will cause a resistance to motion.

 

How do I calculate friction?

Newtons third law states that for every action, there is an equal and opposite reaction. So, for the weight of the object below (W), the reaction force is N and this is equal in value to W, but acting in the opposite direction.

The frictional force, Ff is calculated as this reactionary force, N, multiplied by a factor, µ (Greek mu), called the coefficient of friction. In the diagram above a force, F, acts on the weight from right to left; the frictional force, Ff, acts against this force as shown. We will look at values of coefficient of friction, which is dependent on material, below.

 

Types of friction

Or more correctly, types of coefficient of friction. There are two types – static and kinetic. Static is the coefficient before one body is set in motion relative to the other; kinetic is the coefficient after the bodies are in motion relative to each other. As might be expected, the two are not always the same, more force being required to set a body in motion than to keep it in motion.

 

Typical values for the coefficient of friction

The below table gives indicative values for various combinations of materials. This can be used to make estimates for the coefficient of friction applied in the Norgren ELION Electric Actuator Configurator.