Solve the coefficient of friction in a problem involving circular motion

1. Dec 17, 2007

1. The problem statement, all variables and given/known data
If a curve with a radius of 88m is perfectly banked for a car traveling 75km/hr, what must be the coefficient of static friction for a car not to skid when traveling at 95km/hr?

2. Relevant equations
i guess relevant equations would be tan θ = v^2/rg, but its says next to friction not needed?
f=ma or fr=mar
and Ffr = µs x Fn

3. The attempt at a solution

I made a free body diagram and since it said it was banked i set that in the y axis theres no acceleration, so following the book (Giancoli), I got Fn=mg/cos θ. I solved for that and got 95^2/(88)(9.8) = .40 then inverse tan 10.46 or 84.5 for the angle

then in the x axis i got Fn sin θ - Ffr = mar for the sum of the forces. I substituted Ffr for µs x Fn and Fn = mg cos θ. I
I then had Fn sin θ - µs mg cos θ = mar
I divided mass from both sides and divided both sides by m.
I then for ar substituted v^2/r
Then I isolated µs by dividing cos θ and subtracting Fn sin θ
So I then had µs = V^2/r x cos θ - Fn sin θ
I plugged everything in and I got 9015, I know this wrong just by looking at it, the back of the book says .22, I am completely clueless, so if anyone could help, I'd greatly appreciate it. Sorry if it is a little confusing.

2. Dec 17, 2007

chickendude

The centripetal force of the car is caused by friction

The car is accelerating at $$a = \frac{v^2}{r}$$

Equating the force of friction and the centripetal force, we have

$$F_f = \mu F_N = ma$$

$$\mu * mg = \frac{mv^2}{r}$$

$$\mu = \frac{v^2}{gr}$$

3. Dec 17, 2007