# Homework Help: Simple forces problem

1. Mar 25, 2005

### nerdonsite

ok, our class got an assignment where we have to derive an equation for coefficient of static friction between our text book (inclined at an angle theta) and a coin. The vale of the angle and the mass of the coin are not given. I have come up with the following equation and need to know if it is correct.

u = Ff(static)/Fnormal
*using components of the force of gravity, i determined Ff to be FgxSINxTheta and Fnormal to be FgxCOSxtheta
so,
u = FgxSINxTheta/FgxCOSxtheta

does this seem correct?

thanks for looking.

2. Mar 25, 2005

### tony873004

$$mg sin\theta$$ is the gravitational force down the ramp, so
$$-(mg) sin\theta$$ is the force of static friction.

$$mg cos\theta$$ is indeed the force normal

$$\mu=\frac{F_{friction}}{F_{normal}}$$

$$\mu$$ must be positive, so
$$\mu=\mid \frac{-(mg) sin\theta}{(mg) cos\theta}\mid$$

Which looks exactly like what you got except you call it Fg and I called it mg

3. Mar 25, 2005

### nerdonsite

thanks, i just asked a friend and he got the same thing, so this must me right