IIT JEE , AIEEE Forum - Physics , Mechanics

Raghudevan

does friction depend on surface area, if not why?. it written every where that friction is due to the force of attraction between the molecule that are in contact between the body that is moving and the surface on which it is moving. So, friction should depend on surface area right?

swashata4iit

Well Raghu ur answer goes like this
The co-efficient of friction

depends upon the surfaces in contact. For a particular surfaces in contact the

remains costant.
So

= f(chemistry of surface)

But for a particular surface in contact

= constant
N = normal rxn on the body moving or tending to move = Wt of the body = const(provided that the surface is horizontal)
therefore the frictional force
F =

.N
Since N

f(area)
hence F

f(area).

And also for your quwery friction is not only due to inter molecular force of attraction between the molecules bot it is also due to the unsmoothness of the surfaces. And it has also been proved(which is obviously beyond our scope) that

for a particular suface remains constant and F varies as N hence the net frictional force does not depends upon the area.
Hope its now clear 4 u. Plz rate my efforts.

prakhar_galaxy

hey yaar the correct thing is dat wen two surfaces r in contact then there is attraction b/w the molecules of both surfaces but u will be shocked to know dat only 1 in 1,00,000 particle is in attraction with the other surface (it is proved scientific data)

So wen we talk about two bodies like 2kg body of 1m(square) syrface area & 2kg body of 2m(square) surface area ,den the frction force for both remain d\same coz the no of particles which r in attraction remains nearly same but wen we talk about 2kg body of surface area 1000m(square) ,den definitely the friction force will change
but as we don't talk about such bigger bodies so we consider dat frictin force doesn,t depend on area

I think now it's clear to u

rate me if yes

Betu

the frictional force is NOT because of molecular attraction.

the frictional force is due unevenness of the surfaces which makes some part of one body goes into another and vice versa. Now , greater the force applied between the surface , greater is the depth through which both the bodies hold each other (if you look at magnified view of the contact of surfaces, it will look like meshing of gears)

therefore greater force (normal reaction , say)holds these gears (!) more tightly ; hence greater the opposition of relative movement ....this greater opposition to movement is friction..

it has been seen (yes, only observed..no analytical proof) for two surfaces , friction is prortional to normal force which surfaces apply on each other..

hope it helps..

pl rate me

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