I'm having trouble with the following problem: If a curve of radius 80.0 m is perfectly banked for a car moving 70.0 km/hr, what must be the coefficient of friction in order to prevent skidding when the car is moving at 90.0 km/hr. OK, I know that I need to find the angle of the "perfectly banked" curve first, and I am able to do this. However, what do I do with this angle? It seems that there are too many unknowns. I know that tan(theta)=v^2/Rg, where r is the radius of the circle. But I'm not sure where to go from here. I would really appreciate any help with this problem. Thanks.