I'm bypassing the normal questions, only because I have already solved the question, but want help figuring out why the answer is what it is (without incorporating numbers into it at all). The idea is that a car is at the top of a hill going a constant speed. We are to treat the hill as having a radius, allowing us to use the centripetal force to find out the normal force. My diagram I drew out shows a normal force acting upward on the car, and both the gravity of the car and the centripetal force of the car acting downward toward the radius of the hill (since the car is at the very top). I looked around and was able to figure out the following equation based off a similar question asked on Yahoo... Fn: Normal force, Fg: Gravity, Fr: centripetal force Fn = Fg - Fr What I can't figure out is WHY this is the case. I know it's the correct equation because it led me to the correct answer, but I was under the impression that if the normal force was a reaction of the road to the force of the car, then the normal force would be a sum of the gravity and the centripetal force, not a difference. Why are we not adding the two downward forces, and saying that the normal force is the negative of the two downward forces? Thanks in advance.