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Coefficients of Friction

Friction exists between two solid surfaces, even the smoothest of surfaces are quite rough when you look at them on a microscopic level. Friction even exists when rolling a perfectly round object along a smooth surface, although the amount of friction present is low.

In some instances friction can be seen as a force which can be problematic and is reduced as much as possible, for instance within a car engine it is vital that the engine remains lubricated with oil at all times to help prevent friction from causing wear, oil being a frequently used substance to reduce friction.

On the other hand friction can be very useful and is maximised as much as possible, the brake pads on a car use a material which has a good balance of friction to wear rate which helps stop heavy loads in short distances, not forgetting the tyres which also need to produce a great deal of friction on the road surface when cornering and braking.

Different materials can produce a different magnitude of friction this term is called a coefficient of friction.

Friction Coefficients Table

This table is meant only as a guide.  For any specific application the ideal method of determining the coefficient of friction is by trials.

Surface A Surface B Coefficient of Static Friction μs Coefficient of Kinetic Friction μk
Aluminium Aluminuim 1.05-1.35 1.40
Aluminium Mild Steel 0.61 0.47
Cast iron Cast iron 1.10 0.15
Copper Mild Steel 0.53 0.36
Glass Glass 0.9-1.00 1.00
Graphite Graphite 0.10 0.10
Mild steel Brass 0.51 0.44
Mild Steel Mild Steel 0.74 0.57
Rubber Conrete 1.0 0.80
Teflon Teflon 0.04 0.04

Static Friction

In short to initiate movement in a stationary object you must put in enough energy to overcome the static friction which is holding it still.

Static friction is a force parallel to the two opposing surfaces that are stationary. For example, static friction can prevent an object from sliding down a sloped surface. The coefficient of static friction, typically denoted as μs, is usually higher than the coefficient of kinetic friction as can be seen in the table above. This means it would appear harder to initiate motion of a stationary object than to maintain motion of an already moving object. Why is this? Well on a molecular level the molecules of the two opposing surfaces act to oppose the movement, but once movement has initiated the molecules ability to oppose the movement is reduced.

The maximum possible friction force between the two surfaces before sliding begins is a product of the coefficient of static friction (µs) and the normal force (Fn):

Kinetic Friction

(Kinetic is Greek for "moving")

Sliding friction which is usually called Kinetic friction is a force which opposes motion between two sliding surfaces. Kinetic friction has minimal velocity dependence, meaning that the level of friction will remain the same regardless of the speed of the opposing surfaces.

Next page: Friction on an inclined plane © 2019