A -kg roller coaster car is initially at the top of a rise, at point A. It then moves ft, at an angle of ° below the horizontal, to a lower point B. (a) Choose the car at point B to be the zero configuration for gravitational potential energy of the roller coaster–Earth system. Find the potential energy of the system when the car is at points A and B, and the change in potential energy as the car moves between these points. (b) Repeat part (a), setting the zero configuration with the car at point A.