A coefficient of friction is defined by the relationship between the force of friction between two objects and the normal reaction between the objects that are involved.It is the minimum force required to get an object to slide on a surface, divided by the forces pressing them together. COF is just a number that indicates how difficult it is to get surfaces to slide past each other.

The formula to calculate the coefficient of friction is μ = f÷N. The friction force, f, always acts in the opposite direction of the intended or actual motion, but only parallel to the surface. N is the normal force. Coefficient of friction is dimensionless therefore having no units.

Sajib Aninda Dhar How about the significance of the number? Is it better to get higher COF or less COF?

Thanks for sharing the knowledge.

Thanks for your appreciation and a consequent question. COF is mainly about the shear properties of the solids and of the substances between the surfaces that depends on various factors like contact geometry , applied forces, lubricant chemistry, temperature, relative motions etc. So there is no standard values for COF determination as it depends on many variables. However, the application demands for corresponding COF values. For an example, a brake pad had to have a higher COF value and wear rate based on its application criterion. Rubber tyres on dry concrete possess a kinetic COF value of 1.02 whereas the value is 0.97 for wet condition. Contrastingly, a super-fluid obtains a value as low as zero.

Sajib Aninda Dhar You mention this before, COF is just a number that indicates how difficult it is to get surfaces to slide past each other. Is this mean that the higher COF will make it more difficult to accelerate?

i.e. :

Rubber tires A with COF of 1.00 is better than tires B with COF of 0.80 on dry weather.

But, it better to use tires B when the weather is the wet condition?

Thinking on a practical scenario, Tyre A with higher COF is a better option in wet conditions.

In the general (macroscopic) understanding of friction outlined in the previous answers it is important to distinguish between sticking (the surfaces in contact are not in relative tangential motion to each other, for example in ideal rolling of a wheel or a tyre) and sliding (the surfaces are in relative tangential motion).

Under sticking conditions the friction force is a reaction force resisting motion, that means, the value of the force is exactly the one required to inhibit relative tangential motion. The (static) coefficient of friction \mu_s gives the maximum reaction force, \mu_s*N, before the surfaces start sliding.

So, a higher coefficient will allow you to apply a larger tangential force; in the tyre example you wish to have a higher coefficient of friction to get a higher torque "on the street".

Under sliding conditions the friction force opposing the relative motion is fixed at \mu_d*N (the dynamic coefficient of friction \mu_d often has a slightly smaller value than the static one). In sliding the friction force is dissipating mechanical energy to heat and wear, which in many applications will be undesired. That is why sliding surfaces are often lubricated to reduce the coefficient of friction.

PS: a COF close to zero is referred to as superlubricity, not superfluidity. Infact, the linguistic "similarity" to quantum effects like superfluidity and superconductivity is quite misleading.

Emanuel Willert Sajib Aninda Dhar Many thanks to both of you. Sliding and sticking is 2 condition that we might think deeper when applied practically.

1. COF is unitless.

2. Many factors related to friction either sliding or sticking need to consider before deciding the better COF.

Good Day;

well.

*/ The coefficient of friction is a numerical indicator of how difficult it is to move an object relative to each other.The coefficient of friction relates to two bodies.

*/ Physically, the coefficient of friction represents a dessipation of energy.

For example, to move a 1 kg iron on a glass surface, a certain amount of energy must be available.To carry out the same experiment on a wooden surface, a higher amount of energy must be available.

Excess energy (wood case) is called energy dessipated due to friction.

I hope I have been able to deliver the idea.

Hi Hamid Tebassi, thanks for sharing.

1. Yes, exactly the idea of a simple analogy is that.

2. We want to define COF by giving a force of the same 1 kg iron with different contact material. What the prediction of COF from your example?

3. Is that mean COF also can describe the energy that dissipated due to friction?

Thanks

1. First, there is no ideal contact in nature. All contacts leads to friction, and the role of the engineer, for example, in the manufacture of the engine is to reduce friction (loss of energy) by lubrication.

For example, one kilogram of iron requires a force (imaginary and not real) of 10 Newtons. Thus we get the coefficient of friction by dividing the tangential force and the vertical force.

In the case of wood, it is necessary to move the same mass with a force of 20 Newtons. And we get the coefficient of friction in this case in the same way as before.

We can calculate the work of the two forces necessary for motion in both cases.

Then ask the question. Why there is an increase in force for the same mass.

Yes, rough surfaces increase the coefficient of friction, which increases energy consumption to achieve the same goal (moving 1 kg of iron).

One of the manifestations of friction applied is also the so-called (engine efficiency).

We mean that the theoretical engine power is always greater than real power we get, and this is because there is a loss of energy in different parts of the engine moving relative to each other, and we always try to reduce these losses through good lubrication, for example.

Thanks, Hamid Tebassi .

What is the most significant finding when we plot a graph on the experiment of sliding contact?

1. The highest COF then it drops/decreasing smoothly? or

2. The nearest distance when it came to constant COF?

Or maybe you can suggest to me to refer a detail discussion on this from some simple but useful articles?

Thanks again.

Good day

you can sent me your mail; and i sent you same documentations.

Hamid Tebassi ,kindly pls email to [email protected]

Thanks.