A roller-coaster car of mass 1.40 x 10 kg is initially at the top of a rise at point @. It then moves 38.8 m at an angle of 45.0 below the horizontal to a lower point (B) (a) Find both the potential energy of the system when the car is at points @ and B and the change in potential energy as the car moves from point @ to point (B), assuming y = 0 at point (B) potential energy at A potential energy at B change in potential energy (b) Repeat part (a), this time choosing y = 0 at point @, which is another 15.0 m down the same slope from point (B) potential energy at @ potential energy at B change in potential energy