The purpose of this initial ascent is to build up a sort of reservoir of potential energy . The potential energy you build going up the hill can be released as kinetic energy , energy of motion, as soon as you start coasting down the hill. A roller coaster's energy is constantly changing between potential and kinetic energy. The same thing happens in a roller coaster. When the coaster is released at the top of the first hill, gravity takes over.
Gravity applies a constant downward force on the cars.
If the tracks slope down, gravity pulls the front of the car toward the ground, so it accelerates. If the tracks tilt up, gravity applies a downward force on the back of the coaster, so it decelerates. When the coaster ascends one of the smaller hills that follows the initial lift hill , its kinetic energy changes back to potential energy. In this way, the course of the track is constantly converting energy from kinetic to potential and back again. This fluctuation in acceleration is what makes roller coasters so much fun.