1. The problem statement, all variables and given/known data A skateboarder has to do a loop the vertical loop of a roller coaster track. The radius of the loop is 6.53 m, what is the minimum velocity the skateboarder must have to not fall off when he is at the top of the loop. 2. Relevant equations Equation for centripetal acceleration. Ac=v^2 / r 3. The attempt at a solution I know how to get the answer, basically rearranging for v, and setting Ac to 9.8 to counteract the gravity pulling the rider down. However, I need help understanding the concept. Why does setting the centripetal acceleration as 9.8 give you the right answer? In this case that the skateboarder is at the top of the loop, the gravity is pulling down in the same direction as the centripetal force. I'm pretty sure I am misunderstanding the direction of the centripetal force. Basically my question is: Does the centripetal force act in the same direction as gravity in this case?