Understanding friction
Recently I was asked to investigate an issue with an installation that was getting too hot due to excess friction. This can be a common problem, and so it is helpful to understand the factors involved. In this blog I will explain how friction is created and how it can be measured.
Definition of friction
Friction is resistance that is experienced when two materials are rubbing against each other. It is a physical property which exists between any two different materials which resist a force when making contact with each other. Friction is not always a problem and is an important element in creating motion. The force required to make an object start sliding is slightly larger than the force required for keeping the object still. Therefore we make a distinction between static and kinetic friction.
Below is a short list of defining factors which determine the nature of the movement caused by friction.
m = mass [kg] weight of the objects
Fn = normal force [N]: the force that the weight of the material exerts on the supporting surface.
Fa = tractive force [N]: the force that moves the material against the supporting surface (active force).
Ff = friction force [N]: the force with the potential to offer resistance to the movement of the object against the supporting surface (reactive force)
g = 9.81 m/s2 (acceleration due to gravity; used for converting a mass in kilograms into a force in Newton)
Causes of friction
Friction is mainly caused by the roughness of the surface of the objects making contact with each other. In general, the rougher the surface, the more friction there will be. However, when both surfaces become ultra-smooth, friction caused by molecular attraction begins to play a role, often becoming stronger than mechanical friction. This is mainly the case with soft materials, such as rubber and other pliable plastics. Soft materials will become deformed under pressure, and material deformation can also increase friction.
Friction coefficient
The friction coefficient (m) is the ratio between the friction force (FF) divided by the normal force (FN):
The ratio mentioned above is called Coulomb’s law of friction (Coulomb was a French physicist, 1726-1806). Although the friction coefficient does not have a measurement unit (force divided by force), the law can be easily checked. Pull an object (e.g. a book) by means of a spring balance along a clean surface and read the tractive force (FA). Repeat this measurement with a double load (e.g. two books) and the tractive force required will be doubled.
Method of detecting the friction coefficient
By making use of a spring balance (see above), the approximate friction coefficient between a conveyor belt and a specific surface can be easily detected. There is a physically correct method (example a) as well as slightly incorrect but practically applicable method (example b).
a) Example of the physically correct method
Weight of the object m = 1 kg (unit of mass is kg)
Measured tractive force FA = 4.9 N (unit of force is Newton)
b) Example of the slightly incorrect but practically applicable method
Weight of the object m = 1 kg (unit of mass is kg)
Measured tractive force FA = 0.5 N (unit of force is in fact Newton)
General rules
Below are some general rules about friction and simple tests which can be used to experiment with levels of resistance.
Rule 1:
The friction coefficient always depends on both friction partners (material composition, roughness of surface).
Test: Rub various items with identical pressure, e.g. an eraser, a packet of cigarettes, a coin, a conveyor belt sample on various surfaces, e.g. on a table, a window, a piece of coarse fabric – the resistance will feel very different!
Rule 2:
The friction coefficient strongly depends on the surface conditions (cleanliness, moistness).
Test: Rub two items with identical pressure, first with dry surfaces and then with a few drops of water between the contact surfaces – you will feel the difference in resistance!
Rule 3:
The friction coefficient is not related to the size of the contact surfaces between the two objects..
Test: Pull items of the same material and weight but with different basic surfaces with a spring balance across an identical surface – the tractive force will be the same!
As you can see, friction can become a quite technical topic, but I hope this blog has helped simplify some of the basic factors. If you have any additional questions concerning the subject, please don’t hesitate to contact us and we will be happy to supply you with any additional information.
