Does static friction coefficient change when max friction isn't used?

[Femme_physics]

I know that fsN (static friction x Normal force) is max static friction that can be applied on a force. But when max isn't used, does that mean the friction coefficient is lower?

 

[Studiot]

No the coeficient does not vary,

The formula

F = $$\mu$$R

only holds for a body about to slide.

This is always specified in questions for this reason.

Say we have a 10KN weight sitting on a surface and $$\mu$$=1 and we push it with a force of 5KN.
The weight does not move since the opposing force of friction is 5 KN.

In this situation $$\mu$$ is not equal to 1/2. It is still = 1, we cannot use the formula so we have to know the horizontal force by other means.

Hope this helps

 

[Femme_physics]

Ahhhhhhhhhhh...helps a bunch. Mercy.

 

[Sankalp Sethi]

Static friction is a self adjusting force , if the net force applied in the line of motion is sufficient to allow relative motion between the surface and the body , only then the maximum friction acts on the body that is kinetic friction (coefficient of friction x normal reaction ) . If the force applied in the line of motion is less than what is sufficient to allow sliding then friction force equal to the magnitude of the applied force acts on the body , so as to keep the body at rest.

Sankalp Sethi.

 

[Studiot]

Hello Sankalp, welcome to Physics Forums.

If the force applied in the line of motion is less than what is sufficient to allow sliding then friction force equal to the magnitude of the applied force acts on the body , so as to keep the body at rest.

If a body is at rest, and therefore has no motion, how can it have a 'line of motion' ?

 

[Femme_physics]

Perhaps San refers to this graph?

[PLAIN]http://img64.imageshack.us/img64/2187/staticfriction.jpg

 

[Studiot]

Hello Dory,

Static friction is a self adjusting force

I realized that you understood this (and more) from your original post.

I know that fsN (static friction x Normal force) is max static friction that can be applied on a force.

Whilst the above is true, Sankalp has made quite a complicated statement that could lead to future difficulties.

In particular referring to a 'line of motion' is inappropriate for a body at rest.

It is even difficult for some bodies in motion.

What, for instance is the line of motion of two friction gears meshing together? That is two wheels, one rolling round the circumference of the other.

Or the motion of one of the wheels of a car compared to the car as a whole?

Or say you have a bicycle with a sprung front wheel, going over a bump. What is the line of motion of that wheel compared to the line of action of the force of friction?

It is far better (and easier) to keep discussion of 'lines' to lines of action of the forces involved. These will always work out, whatever the motion of the system.

 

[Sankalp Sethi]

I think Studiot is correct , we cannot define a line of motion incase a body is at rest , it should be line of action , sorry , wrong choice of words. Except that , i feel that my explanation is correct , isn't it?

 

[Studiot]

Except that , i feel that my explanation is correct , isn't it?

Yes you had the right idea.