Friction depends upon area or not?

[Ahsan2011]

My books say that friction is independent of the area of contact, but only depends upon the coefficient of friction and the weight of body. but well according to my science, there is something wrong with it. because you observe, take same masses but different geometeries for example rectangular area of contact and other having cubic area of contact. then what actually happens the coefficient of friction doesn't remain same i did it in my home. frictional force comes out to be greater. what's this?
if it does not depend upon area of contact why we have wheels?
please some body help me

 

[Delta2]

The coefficient of friction depends only on the material of the contact surfaces. I suppose when you did some experiments with different geometries the material was different too (and maybe the weight too).

Another factor would be the height of the surface, theoretically height should be zero but in reality the objects have some height. This might alter the kinetic behaviour of the object as you apply some force to it and making it to rotate instead of just going forward and thus making you think that the friction is different.

 

[rcgldr]

My books say that friction is independent of the area of contact.

That's an idealized explanation. In the real world friction is more complicated than that. A 4:35 into video #2 at the website below, 4 small blocks of different sizes on a flat sheet are shown to start sliding at different tilt angles, with the smallest block sliding last.

http://www.gyroscopes.org/1974lecture.asp

In the case of tires, maximum static friction force does not increase lineary with normal force, this effect is called load sensitivity:

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

 

[Delta2]

That's an idealized explanation. In the real world friction is more complicated than that. A 4:35 into video #2 at the website below, 4 small blocks of different sizes on a flat sheet are shown to start sliding at different tilt angles, with the smallest block sliding last.

http://www.gyroscopes.org/1974lecture.asp

I think this happens because of friction coefficient not being exactly the same but varying across the contact surface. The greater the surface , the greater the result of this inhomogenity will be.

 

[Lsos]

Friction is not a simple or fundamental force by any means. There are different mechanisms behind frictional force which grow or weaken depending on many factors (normal force, velocity, lubrication, materials, temperature, area, etc. etc) and even then not all of them are understood.

Therefore, saying that friction does not depend on surface area is a gross oversimplification at best. However, for one paragraph in an introductory physics book, in a specific case with many assumptions, this explanation of Friction = normal force x coefficient of friction appears to be adequate. Just be aware that this "coefficient of friction" can change, morph, shift, curve, or even reverse depending on many factors.

It's kind of like explaining a woman with the formula "Woman = Human x coefficient of womanly behavior". It's great, but it's meaningless without knowing what the "coefficent of womanly behavior" is, and it can be ANYTHING.