An engineer wants to design a circular racetrack of radius L such that cars of mass M can go around the track at speed U without the aid of friction or other forces other than the perpendicular contact force from the track surface.

Suppose the race cars actually round the track at a speed w>U. What additional radial force Fr is required to keep the cars on the track at this speed? Express the answer in terms of M, L, U, w, and g.

There is a question like this on but it doesn't have the right answer.