A person drives a car over the top of a hill, the cross section of which can be approximated by a circle of radius r=250m. What is the greatest speed at which he can drive without the car leaving the road at the top of the hill? I know that the maximum speed occurs when the normal force= zero. And by my calculations vmax=√rg (where r is the radius and g is the acceleration of gravity) Is this all I need? Am I missing anything?