# Homework Help: Friction help

1. Nov 13, 2006

### BigMann

stupid question because I forgot the equation

What is the equation for the minimum value of the coefficient of friction?

is it m(v^2/r) or v^2/gr

2. Nov 13, 2006

### physics girl phd

check your units on them -- think about the units of the coefficient of friction -- then does one or the other make sense?

3. Nov 13, 2006

### BigMann

the correct equation is v^2/gr right?

4. Nov 13, 2006

### physics girl phd

well -- what are the units of that, and what are the units of the coefficient of friction?

5. Nov 13, 2006

### BigMann

I know that the coefficient of friction is Newtons

6. Nov 13, 2006

### physics girl phd

7. Nov 13, 2006

### HallsofIvy

It's what we "know" that hurts! The Newton is a unit of force. While friction is a force, the coefficient of friction is not.

8. Nov 13, 2006

### BigMann

This then means that the first equation would have to be the right one based off of the units. And if Im incorrect than I guess I just forgot it all

9. Nov 13, 2006

### physics girl phd

The units of the first are mass x velocity^2/length -- kg/m^2/s^2/m -- is that the same as the units of the coefficient of friction?

Then analyze the second in the same way.

Analyzing units always helps -- but of course doesn't guarantee the right answer.

10. Nov 13, 2006

### BigMann

maybe I'm not understanding the units of the coefficient of friction. Could you explain to me what they are?

11. Nov 13, 2006

### OlderDan

The coefficient of friction is the ratio of two forces, the force of friction divided by the normal force. It has no units; it is a pure number.