Have you ever gotten on a roller coaster and noticed that after the car was carried up the first hill, there were no more motors to speed you up? In addition, did you notice that there were no brakes to slow you down until the very end of the ride? And yet the speed kept changing the whole time. That is the power of mechanical energy.
This chapter examines energy as an alternative approach to modeling motion. Depending on the type of game, this might or might not be more efficient, but it's always good to have options.
The morph ball in Metroid Prime uses this to outstanding effect.