Why coefficient of friction is less than 1

[reza]

why coefficient of friction is less than 1?
dose it have a phisical answer

 

[Gyroscope]

No, it is not.

 

[reza]

then what is the coefficient of friction depends on

 

[FredGarvin]

To an extent, the COF depends on surface roughness of the mating materials. However, you can get to a point where highly polished surfaces exert more friction than would be expected. You see that in some high speed seals and lapped surfaces in hydraulics.

The range of COF can span both >1 and <1. Here are a couple of references:

http://www.roymech.co.uk/Useful_Tables/Tribology/co_of_frict.htm
http://en.wikipedia.org/wiki/Friction
http://www.school-for-champions.com/science/frictioncoeff.htm

If you do a google search, there are tons of references.

 

[reza]

thank you very much

 

[engineroom]

A cooefficent of 1 is considered perfect and the mating surfaces are hence frictionless. If an object was pushed across a surface with a coefficient of 1 then the object would continue to slide forever.

Any cooefficent is always unitless.

 

[C4guy]

Hi!

A cooefficent of 1 is considered perfect and the mating surfaces are hence frictionless. If an object was pushed across a surface with a coefficient of 1 then the object would continue to slide forever.

Errr... That would be a friction coefficient of 0.

c4guy

 

[reza]

if (coefficient of friction)=1 we can say just f=N.(COF) => f=N and some times f=m.g

 

[pixel01]

When you magnify the surface of the objects, it is surely not completely flat, but rough. From that viewpoint, I model it like pyramids (see picture). So with the model you can calculate the friction coefition. If the angular of the pyramid (at bottom) equals 45 degree, the coefition is 1.
It is easy to see that it's very difficult for a pyramid with bottom angular of more than 45 degrees. That's why as you can chech in the friction table, most of the materials have the friction coefficient of lower than 1 or approx. to 1. Of course this depends on the crystal structure of the material.
The friction I mention above is the dynamic, not the static one. For static friction, there may be some more force linked with the adsorption force or else..

 

[DyslexicHobo]

With a coefficient of friction greater than 1, you can assume that it requires less force to lift the object from the ground, rather than slide it across a surface.