Okay so i have a test and so I'm reviewing for it, and one of the questions i came across was: 1. A curve of radius 180 m is being designed in a new highway to allow cars travelling at 70 km/h to round the curve with zero frictional forces. a) At what angle must the road be banked? b) What is the minimum coefficient of friction that will allow a car to successfully go around the curve at 30 km/h? so i did a, i used Fn = mg/cos theta, and Fn (sin theta) = mv^2/r and substituted for Fn, getting Theta = Tan inverse v^2/gr and i got an angle of 12.1 degrees which is right. But I'm having a problem getting friction. i tried to assume a mass but that didn't work, and I'm not sure how to start this question. any ideas?