# Homework Help: Help, annoying friction equation

1. Jun 3, 2009

### The riddler

I have found an equation in a physics text book im reading that leaves me completly stumped. It is about the co-efficient of friction, looks very simplistic but when i read it this equation looks very illogical. I probably don't understand it fully or something like that but I really need someone to please explain exactly how this equation works.

The equation represents the co-efficent of friction, it looks simple enough but i have some concerns with it.

u=Co-efficient of friction F= Force of friction R= Force pressing surfaces together
u=F/R

My problem with this equation is that its states that if the force pressing the surfaces together increases (say the force of gravity on a moving car increases) then that makes the Co-efficient of friction less as the force of friction would be divided by this increased force. But surely thats not true because if the force of gravity increased then logically the co-efficient of friction would increase with it.

2. Jun 3, 2009

### cepheid

Staff Emeritus
There is the fallacy in your interpretation. The coefficient of friction is meant to be interpreted as a CONSTANT (capital letters for emphasis only, not shouting). It is a parameter that depends only on the material properties of the two surfaces. Really it is a way for us to characterize friction without delving too much into the microscopic details of what is going on (on an atomic level) to make friction arise. Without understanding any of that, we can still state this simple, elegant relation that summarizes what we will observe (macroscopically): namely that the friction force for the two surfaces in contact is PROPORTIONAL to the force with which they are being pressed together (the normal force). Mu is, in this case, the constant of proportionality.

So the proper way to apply this equation to your example is as follows: If the gravitational force on the car increases, then the frictional force between the tires and the road must ALSO increase by a corresponding factor, since their ratio is a constant, mu, which is the coefficient of friction between rubber and asphalt (or whatever). Simply put, the more weight you have, the more traction you have.

3. Jun 3, 2009

### Chrisas

Try this. Rest you hand lightly on a smooth table. Now you should be able to slide it fairly easily. Now put your other hand on top. Start pushing harder and harder (more force) on the sliding hand. You should notice it takes more effort in your arm to move your hand across the table, the harder you press down on it.

4. Jun 4, 2009

### The riddler

Oh, ok i get it now thanks for your help.