# Does friction depend on area of surface of contact?

1. Aug 7, 2007

### ank_gl

dont have much time, ll make it quick

does friction depends on area of surface of contact??
i dont think it does

2. Aug 7, 2007

### phy-79

well actually, yes.

3. Aug 7, 2007

### phy-79

an example for this is seen in cars.When the area under contact is more, in tyres, the milage(kms covered per litre of petrol) of the car reduces as more friction acts on the tyre,thus resulting in the need of more power for attaining a particular speed.

4. Aug 7, 2007

### ank_gl

exactly thats what came to my mind at first
but frictional force is coefficient of friction * normal force. mu is constant, normal force is m*g. neither of the three quantities change with using broader tyres, do they?? NO, they dont. so how frictional force depend on area.

5. Aug 7, 2007

### mgb_phys

Friction is due to the microscopic bonds forming between the atoms of two surface close together. The more area you have in contact the more atoms, more bonds, more friction.
It also depends on the amount of deformation of the materials - more pliant materials deform to out more surface are in contact. That is one of the reasons that inflatable tires are sued on cars and jewels are used in watches.
Finally it depends on the chemical nature of the two materials and the strength of the bonds it forms.

6. Aug 8, 2007

### ank_gl

that i know, but how do you negate the fact that i talked about in post#4.
frictional force = mu*normal contact.
none of the two change by increasing the area.
practically speaking, true, cars with broader tyres have better grip, an example is formula 1 cars, but how, or more precisely, why??

7. Aug 8, 2007

### AlephZero

In your post #4 you are not talking about friction, you are talking about the Coulomb model of friction.

Coulomb's model (or so-called law) works fairly well for in some situations. Because of the way mechanics is taught it schools, some people get the idea that it applies to every situation that could be described as "friction", but it doesn't.

It doesn't apply at all for many combinations of materials. For, example, if surfaces are coated with adhesive there can be a "friction force" even when the normal load is negative!

Car tyres are just one example where the Coulomb friction model isn't adequate to explain what you observe. And since Coulomb died in 1806, he certainly never thought about the friction of synthetic rubber on asphalt when he proposed his "law".

8. Aug 8, 2007

### ank_gl

well any body can be considered as a join of two saperate bodies, therefore your concept of negative loading is plain stupid

so what is the law goverining friction which covers the term of area??? just the point that coulomb didnt thought of this situation, doesnt make his law invincible. if a law is failing at a point, it should be scrapped or should be conditionalised.
as far as teaching of the law in school, we did study about the car example and DID use the same formula.
i am not trying to defend my point, i know that friction depends on area, but just this formula is not getting into my head. according to it, frictional force depends entirely on weight(normal force), not on area

Last edited: Aug 8, 2007
9. Aug 8, 2007

### mgb_phys

'mu' is not a universal constant of the materials but only constant for that experimental setup. Even then it varies if for instance the surface is able to deform under the normal force or if the material changes with temperature.

10. Aug 8, 2007

### ank_gl

hmmm that might be a possibility..
so the coefficient of friction depends on area of contact??
i very well know the fact that friction DOES depend on area, the microscopic bonds explanation being a good enough reason. i am having this debate with some of my friends & i am on the
"AREA affects FRICTION" side. i just didnt give the coefficient of friction varying, much thought

Last edited: Aug 8, 2007
11. Aug 8, 2007

### ice109

12. Aug 9, 2007

### ank_gl

13. Aug 9, 2007

### kach22i

14. Aug 9, 2007

### ank_gl

geez kach, you are mad for hoverthings!! arent u??