The coefficient of friction is a constant value that defines the resistance to motion due to friction. It can also be defined as the ratio of frictional force to the normal force. It is a unitless factor that allows us to comparatively define the friction between a pair of materials. It is a scalar quantity.
The value of the coefficient of friction is different for each pair of objects. That means friction depends both on the sliding object as well as the sliding surface. For more details, you can read our post on factors affecting friction. The value of the coefficient of friction ranges from zero to slightly greater than one. The higher the roughness higher is the value.
The formula for the coefficient of friction
μ = f/N where μ is the coefficient of friction, f is the frictional force and N is the normal force acting at the point of contact.
Classification of the coefficient of friction
The value of the coefficient of friction will be different for different cases which can be classified as follows.
- Based on the state of motion: Coefficient of static friction and coefficient of kinetic friction
- Based on the mode of contact: Coefficient of rolling friction and coefficient of sliding friction
The coefficient of kinetic friction always has a value lesser than limiting friction.
The value of the coefficient of static friction is self-adjusting.
The coefficient of sliding friction is always greater than the value of the coefficient of rolling friction
What is the coefficient of limiting friction?
The limiting coefficient of friction is the maximum value the coefficient can attain while the body is at rest. This can be understood by looking at the friction plot. In the case of friction in a slope, it can be calculated as the tangent of the angle of friction.
What is Amonton’s law of friction?
Amonton’s law of friction states that the frictional force is directly proportional to the normal load. It indicates that the ratio of frictional force to normal force is constant. It is a classical version of the law of friction given by Guillaume Amonton in 1699.
Amonton’s second law states that friction is independent of the area of contact. The friction mentioned here is actually dry friction.
Examples of the coefficient of friction
Examples of high coefficient of friction
- Sliding on a rough wooden surface
- Rubber on paper
- Walking with high-grip shoes on the pavement
- sandpaper against a wooden surface
- Sharpening iron on copper
Examples of low coefficient of friction
- Walking on ice
- Grease on steel
- Oil on wooden surfaces
- Walking on marshy surfaces
Walking on thin ice is a metaphorical warning which means you are in danger or at risk of something. It is due to such a low coefficient of friction between our feet and friction that we slip and fall down.
You can see here the values of the coefficient of static and kinetic friction of the most commonly used material pairs.
You might have observed that the lubricants and slippery materials have a coefficient of static friction as low as 0.05.
A combination of rough and smooth material results in a moderate value. Examples include aluminum on snow, glass on nickel, and leather on iron.
And, rougher materials like iron on copper have higher values like ~1. Other examples include brass on a steal, metal on metal, Plexiglas on Plexiglas, etc.
You might have also noticed that the surface conditions and the temperature also affect the coefficient of friction. Surface conditions like clean or dry and grease or lubrication can affect the value of the coefficient of friction significantly. The temperature affects materials like water which has a change in the state of matter at low temperatures.
What is the apparatus used to measure the coefficient of friction?
Tribometers are used to measure the coefficient of friction and other friction-related properties like wear rate etc. Nowadays motorized tribometers are available which can measure dynamic values of friction.
The coefficient of friction can also be calculated using the basic classical laws of physics. Some of the methods are listed below.
- Spring balance
- Tilted plane
- Clamping
- Pendulum
You can read this article with DOI 10.24874/PES01.01.004 to learn more about tribometers.
Does the weight affect the coefficient of friction?
The coefficient of friction does not depend on the weight of an object. It only depends on the nature of the materials. The irregularities on the surface, the charges on the materials, and other factors can affect the coefficient value.
