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B How comes friction is not contact surface dependant?

  1. Dec 10, 2016 #1
    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

    this seems contradictory with coulomb

    is still coulomb view mainstream or has it been corrected?
  2. jcsd
  3. Dec 10, 2016 #2


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    Do you mean contact surface area or the properties of the materials in contact ?
  4. Dec 10, 2016 #3
    no i mean contact surface area, coulomb clearly set it this way to be perpetuated for centuries by the authority criteria
  5. Dec 10, 2016 #4

    Vanadium 50

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    Nothing you have written is correct. Nothing you have written is even a sentence. Perhaps you could try again, showing more care.
  6. Dec 10, 2016 #5
    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?
  7. Dec 10, 2016 #6

    Vanadium 50

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    i guess that since you dont 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 dont 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 dont i am sorry if the answer is not more detailed but since you dont seem to write down a clear and detailed question this is the best i can do for now
  8. Dec 10, 2016 #7
    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."
  9. Dec 10, 2016 #8
    here the 3 laws of dry friction i dont 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:
    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.
  10. Dec 10, 2016 #9


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    Rather obviously English is not the OPs native language. Shall we all try to respond perfectly in his native language?
  11. Dec 10, 2016 #10

    Vanadium 50

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    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.
  12. Dec 10, 2016 #11
    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.
  13. Dec 10, 2016 #12
    " 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
  14. Dec 10, 2016 #13
    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.
  15. Dec 10, 2016 #14
    I feel as if the OP should have researched more before asking a question in a way that deterred people from answering.
  16. Dec 10, 2016 #15
    Then i understand that tyre gripe doesnt go in function of friction force since tyre grip is indeed contact surface area dependant

    is this right?
  17. Dec 10, 2016 #16


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    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: Dec 10, 2016
  18. Dec 11, 2016 #17
    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
  19. Dec 11, 2016 #18


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    If the normal forces have not changed, and the static/dynamic friction values have not changed, then the situation has not changed.
  20. Dec 11, 2016 #19
    yes i know but dont 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 dont know cause we have nothing better to compare to

    maybe antonon wouldnt 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
  21. Dec 11, 2016 #20


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    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).
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