If a car is going up or down an inclined plane, the normal force on the car is just mg cos (theta). However, if a car is turning on an inclined curve, the normal force is mg/cos(theta). Why are the two normal forces not the same? To clarify, here is an example problem: A highway curve of radius 70m is banked at a 15 degree angle. At what speed can a car take this curve without assistance from friction? I've attached a force diagram of the problem. Thanks for the help!