How comes friction is not contact surface dependant?

• farolero
In summary, according to Coulomb, the force of friction is independent of the contact surface area. However, this seems contradictory with the way tyre grip works, as it is dependent on the contact surface area. It is unclear if Coulomb's view is still mainstream or if it has been corrected. Additionally, the three laws of dry friction state that friction is directly proportional to the applied load, independent of the apparent area of contact, and independent of the sliding velocity. However, the real Coulomb law states that the critical shear force per unit contact area is equal to the normal force per unit contact area times the coefficient of static friction. This means that friction is actually dependent on the normal and shear stresses, rather than the contact area. As
farolero
according coulomb force of friction is independant of contact surface

a tyre car grip goes in function of contact surface and grip is caused by friction

is still coulomb view mainstream or has it been corrected?

farolero said:
according coulomb force of friction is independant of contact surface

Do you mean contact surface area or the properties of the materials in contact ?

no i mean contact surface area, coulomb clearly set it this way to be perpetuated for centuries by the authority criteria

Nothing you have written is correct. Nothing you have written is even a sentence. Perhaps you could try again, showing more care.

according my physics classes notes coulomb discovered and stablished force of friction is not contact area dependant

how can this be explained by the way tyre grip work being contact are dependant and being grip friction dependant?

i guess that since you don't want to write proper english it must be because you find writing in this style easier so i will be sure to use it so it will be easier for you to read i don't understand exactly why this is the case but it seems that it is so her i go anyway what you should do is first read the wikipedia article and then write down coulombs frictional law and see which parts of it you expect to be constant and what parts you don't i am sorry if the answer is not more detailed but since you don't seem to write down a clear and detailed question this is the best i can do for now

billy_joule, jbriggs444 and Dale
farolero said:
according my physics classes notes coulomb discovered and stablished force of friction is not contact area dependant

how can this be explained by the way tyre grip work being contact are dependant and being grip friction dependant?
Because the real Coulomb law is that "the critical shear force per unit contact area (i.e., shear stress) is equal to the normal force per unit contact area (i.e., normal stress) times the coefficient of static friction."

davefarrell60
here the 3 laws of dry friction i don't see units of area mentioned anywhere what i understand from there is that friction of tyres or grip is independent of the apparent area of contact:
from:
The elementary property of sliding (kinetic) friction were discovered by experiment in the 15th to 18th centuries and were expressed as three empirical laws:

• Amontons' First Law: The force of friction is directly proportional to the applied load.
• Amontons' Second Law: The force of friction is independent of the apparent area of contact.
• Coulomb's Law of Friction: Kinetic friction is independent of the sliding velocity.

i guess that since you don't want to write proper english it must be because you find writing in this style easier
Rather obviously English is not the OPs native language. Shall we all try to respond perfectly in his native language?

Tom.G said:
Rather obviously English is not the OPs native language

I have no problem with the sorts of errors caused by that. I do have a problem with a) someone who won't write complete sentences, b) provide incomplete information, and c) still provide incomplete information when called on it. If someone has shown on other threads that they can write clear sentences if they wish, that means that if they don't, it's by choice.

Chestermiller said:
Because the real Coulomb law is that "the critical shear force per unit contact area (i.e., shear stress) is equal to the normal force per unit contact area (i.e., normal stress) times the coefficient of static friction."
I stand by what I said in post #7. It is really the normal- and shear stresses that matter. If these are uniform over the contact area, then the relationship between the friction force and the normal force is independent of the contact area.

Nidum
" the friction force and the normal force is independent of the contact area"

then assuming a tyre grip goes in function of their force of friction its indiferent to wear big tyres or small tyres concerning grip

maybe the advantage of big tyres then its they accumulate less heat and work more time at perfect temp

i google it and this confuses many people, is something antiintuitive

The reason that friction is independent of the surface area is because if you increase the surface area while maintaining the same amount of force, you decrease the pressure (which is the force per unit area). This leads to the normal forces being X times smaller per unit area, while having X times more area, leading to the same amount of friction regardless of surface area.

Additionally, although this is a forum and not English class, please try to write using proper grammar and punctuation.

I feel as if the OP should have researched more before asking a question in a way that deterred people from answering.

Then i understand that tyre gripe doesn't go in function of friction force since tyre grip is indeed contact surface area dependant

is this right?

@farolero

The so called Laws of Friction only really work for dry surfaces , simple geometries and simple loads .

Determining the effective friction forces acting in a tyre / road surface grip interaction is complicated and requires evaluation of several different influencing factors .

To understand friction properly start at the beginning . Get to understand the action of friction between a simple loaded block and a flat table . When you have mastered that move on to more complicated situations .

Last edited:
imagine this problem:

theres a formula one car with the brakes on in a 45º ramp, the static friction value is 1.3 and dynamic friction value is 1.1

now you change the tyres of the car by a bycicle ones with same materials and hence same friction values than before

has the situation changed in what case will the car move down the ramp?

i would like to know though would never be the case, if this was an exam what would be the right answer, it feels bad knowing telling the truth will make you fail

If the normal forces have not changed, and the static/dynamic friction values have not changed, then the situation has not changed.

yes i know but don't you think its antiintuitive?

if you make the experiment on the problem set and start leaning more and more each platform both cars should drop at the same time

i was expecting an answer on the vein that this model was outdated or simplififed for textbooks but seems totally vigent and mainstream, maybe we live in true dark eras and don't know cause we have nothing better to compare to

maybe antonon wouldn't mind race a grandprix with bycicle tyres

i wonder if next time you buy some tyres and the salesman tells you that the contact surface is indiffirent and that some bycicle tyres will keep your kids safe from accident you must believe him

Agree, the example you presented is very anti-intuitive. If you have some free time, you should do some experiments.

The thing with F1 tires is that they are very "soft", to maximize coefficients of friction. Constructing a bicycle tire from the same material would result in structural failures of the material. Otherwise F1 vehicles would benenfit from the narrower tires (less air resistance).

@farolero

Chestermiller said:
It is really the normal and shear stresses that matter. If these are uniform over the contact area, then the relationship between the friction force and the normal force is independent of the contact area

That is the key to understanding your problem . Can you see why ?

Yes thanks a lot for helping me,I uderstand that part, the less contact surface the bigger the normal force per unit of area and the more contact surface the smaller the normal force per unit of area so they nullify each other resulting overall independent of contact area.

But I fail to see how this could be valid on basic tyre behaviour, I just can not say its too complex and hence shouldn't try to understand it.

farolero said:
Yes thanks a lot for helping me,I uderstand that part, the less contact surface the bigger the normal force per unit of area and the more contact surface the smaller the normal force per unit of area so they nullify each other resulting overall independent of contact area.

But I fail to see how this could be valid on basic tyre behaviour, I just can not say its too complex and hence shouldn't try to understand it.
Well, it's an approximation. In the ideal situation where the tire carcus is treated a membrane, the normal force per unit contact area is constant, and equal to the tire pressure. So, in that situation, the static frictional behavior is independent of the contact area. Of course, in the case of a real tire, the normal force per unit area is not really uniform.

OK I think I understand now:

Antonons third law is for ideal cases and valid for real cases as well as long as the contact surface is flat, but if the surface is curved the pressure for unit of surface is not uniform and hence friction indeed is surface dependant.

Maybe if my teacher had explained me as you did that Antonons law is only valid for ideal cases and not valid for real cases where the contact area is curved i wouldn't have distrust mainstream science ever

farolero said:
OK I think I understand now:

Antonons third law is for ideal cases and valid for real cases as well as long as the contact surface is flat, but if the surface is curved the pressure for unit of surface is not uniform and hence friction indeed is surface dependant.

Maybe if my teacher had explained me as you did that Antonons law is only valid for ideal cases and not valid for real cases where the contact area is curved i wouldn't have distrust mainstream science ever
In a real tire, even though the contact patch is flat (assuming no tread pattern), the normal contact stress is not uniform due to the bending rigidity of the rubber composite.

1. How does friction work?

Friction is a force that resists the relative motion or tendency of motion between two surfaces in contact. It is caused by the microscopic irregularities on the surface of objects that interlock with each other when they come into contact, creating resistance to motion.

2. Why is friction not dependent on the contact surface?

Friction is not dependent on the contact surface because it is a result of the interlocking of microscopic irregularities on the surface of objects. This means that even if two surfaces appear smooth, they still have small imperfections that can create friction.

3. What factors affect the amount of friction between two surfaces?

The amount of friction between two surfaces is affected by the roughness of the surfaces, the force of the objects pushing against each other, and the type of material the surfaces are made of.

4. Can friction be reduced?

Yes, friction can be reduced by using lubricants, such as oil or grease, which create a slippery layer between the two surfaces. Additionally, smoother surfaces can also reduce friction as there are fewer irregularities for the surfaces to interlock with.

5. Is friction always a bad thing?

No, friction is not always a bad thing. In fact, it is essential for many everyday activities, such as walking, driving, and writing. However, too much friction can cause wear and tear on surfaces, which can be undesirable.

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