Aroller coaster car sits at rest at the top of 40 m hill (y = 40 m). the zero line for this problem is located at the bottom of the hill (y = 0). a) what is the change in potential energy as car moves from point a to point b? b) what is the speed of the car at the bottom of the hill? c) what is the speed of the car when it reaches point b? d) the car is released from point a again, but this time it only reaches a height of 25.0 m above the reference point. if the mass of the car is 1000 kg and the car travelled a distance of 400 m, estimate the magnitude of the frictional force between the car and the track.