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Coefficient of Friction and Normal Force

by Abraham
Tags: coefficient, force, friction, normal
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Abraham
#1
Jul5-08, 03:43 PM
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I know that the force of friction is the (coefficient of friction) x (normal force). My question is, why isn't area involved? That is, why wouldn't a larger surface have more friction than a smaller surface, if the normal force is the same?

PS: Sorry, just in case this is in the wrong section
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nicksauce
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Jul5-08, 04:36 PM
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There is no reason why the coefficient of friction can't implicitly depend on area.
tiny-tim
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Jul5-08, 05:49 PM
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Quote Quote by nicksauce View Post
There is no reason why the coefficient of friction can't implicitly depend on area.
erm I've no idea what that means.
Quote Quote by Abraham View Post
I know that the force of friction is the (coefficient of friction) x (normal force). My question is, why isn't area involved? That is, why wouldn't a larger surface have more friction than a smaller surface, if the normal force is the same?
Hi Abraham!

Because the larger the surface, the more its weight is spread out.

The friction per area depends on the pressure between the surfaces *the harder they're pressed together, the more friction you'd expect.

Pressure is force divided by area.

Halve the area, and the pressure is halved, so the friction per area is halved, so the total friction is still the same.

nicksauce
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Jul5-08, 05:59 PM
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Coefficient of Friction and Normal Force

Quote Quote by tiny-tim View Post
erm … I've no idea what that means.
The OP's first point (it seems) was that F = uN, implies that F does not depend on area. I was saying that this was false, as u could implicitly depend on area, as u also implicitly depends on many other things (material, roughness of surface, possibly temperature). You covered the rest, by explaining how F should not depend on area.
chandangang
#5
Sep19-08, 12:12 PM
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Quote Quote by tiny-tim View Post
erm I've no idea what that means.


Hi Abraham!

Because the larger the surface, the more its weight is spread out.

The friction per area depends on the pressure between the surfaces *the harder they're pressed together, the more friction you'd expect.

Pressure is force divided by area.

Halve the area, and the pressure is halved, so the friction per area is halved, so the total friction is still the same.
if area is halved then pressure wud double (force remains same throughout-normal reaction force).
could u put forward ur point properly
stewartcs
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Sep19-08, 03:02 PM
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Quote Quote by nicksauce View Post
There is no reason why the coefficient of friction can't implicitly depend on area.
The coefficient of friction in the standard friction model is dependent on the material properties mainly and to the environment to a certain extent, not the area.

CS
stewartcs
#7
Sep19-08, 03:09 PM
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Quote Quote by nicksauce View Post
The OP's first point (it seems) was that F = uN, implies that F does not depend on area. I was saying that this was false... You covered the rest, by explaining how F should not depend on area.
You're contradicting yourself.

F does not depend on the area.

CS
stewartcs
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Sep19-08, 03:12 PM
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Quote Quote by chandangang View Post
if area is halved then pressure wud double (force remains same throughout-normal reaction force).
could u put forward ur point properly
Please refrain from using text-speak. It makes interpreting your comments difficult.

CS
rcgldr
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Sep19-08, 06:48 PM
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In the real world it depends on the surfaces involved. In the case of tires, maximum static friction force does not increase lineary with normal force, the ratio is called load sensitivity:

http://en.wikipedia.org/wiki/Tire_load_sensitivity

For tires, a larger area reduces the force per unit area, increasing friction (there is a point of diminishing returns due to unsprung weight).

Also in the second half of the second video on this web page, 4 objects of the same density are placed on a smooth board, and the smallest object slides last, although I'm not sure if this is friction force or something related to air between the objects and board.

http://www.gyroscopes.org/1974lecture.asp
Pyrrhus
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Sep19-08, 09:48 PM
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Quote Quote by Abraham View Post
I know that the force of friction is the (coefficient of friction) x (normal force). My question is, why isn't area involved? That is, why wouldn't a larger surface have more friction than a smaller surface, if the normal force is the same?

PS: Sorry, just in case this is in the wrong section
Friction is a complex phenomenon. One explanation of friction in dry surfaces (or very lightly lubricated) was introduced by Coulomb (it is a very simplified explanation, and what you are talking about). The static friction and kinetic friction coefficients are usually found to approximated values on engineering handbooks. Also, as you noticed Coulomb's theory doesn't care for the contact surface, which indeed can affect on the movement of a body in some cases.

In simple words, within its range of applicability Coulomb's theory of dry friction has given good results. For cases where Coulomb's theory is not applicable such as lubricated surfaces, other theories must be employed.
chandangang
#11
Sep20-08, 01:33 PM
P: 5
Now read this..
When the surface area is reduced , the pressure on the contact points between the two surfaces increases. As a result some contact points undergo deformity in shape and hence more number of contact points are obtained (imagine the contact points to be like mountains if the height of peak is reduced then there would be more number of hills with the same height than were previously).
When there is greater surface area , pressure on the contact points are less. So they do not undergo deformity, but, due to greater surface area number of contact points are more already.
So in both the cases the total number of contact points remain same and hence friction remains same in both the cases.


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