A 1 000-kg roller coaster train is initially at the top of a rise, at point A. It then moves 135 ft, at an angle of 40.0° below the horizontal, to a lower point B. (a) Choose point B to be the zero level for gravitational potential energy. Find the potential energy of the roller coaster–Earth system at points A and B, and the change in potential energy as the coaster moves. (b) Repeat part (a), setting the zero reference level at point A.