Imagine you have a level track with a loop de loop on it. The loop de loop is shaped like a perfect circle. You are inside the cart and approaching the loop de loop. I know that gravity and the normal force both act to produce the centripetal force. However at the very bottom of the track the normal force and gravity are in opposite direction if the track is level. Since you start going in a circle at the bottom of the loop there has to be an imbalance of forces. So the normal force at the bottom of the loop must be greater than the force of gravity. My question is how is that possible at the instant you are at the bottom of the loop if the track is level.