# Homework Help: Calculating Friction

1. Jun 18, 2006

### JCCol

I was wondering if someone could help me with calculating friction. I just don't understand how to get the friction coefficents. I was hoping someone could help me or redirect me to another post if this is a repost.

2. Jun 19, 2006

there is a table for friction coefficients. but if you have kinetic friction and have the normal force and the force of the friction then you can do: force friction/normal force = Uk. my physics might be a bit rusty =/

Last edited: Jun 19, 2006
3. Jun 19, 2006

### Hootenanny

Staff Emeritus
Indeed quadruple is correct, if you are able to obtain the firctional force and the normal reaction force, all you need to do is rearrange to give;

$$\mu = \frac{R}{F}$$

Note that R always acts perpendicular to the surface R = mg only holds when the surface is horizontal, otherwise you are required to resolve the force of gravity to perpendicular to the surface.

Alternatively, if you are working with energy, the work done by a frictional force is given by $w = xR\mu$, where x is the displacement, thus the coefficient of friction is given by;

$$\mu = \frac{w}{xR}$$

There are many ways to obtain the coefficient of friction, depending on the circumstances in which it occurs. Perhaps, you could post a specific example of the type of question you are have difficulties with.

Last edited: Jun 19, 2006