The simple answer to how roller coasters work — physics. Along with some mechanical systems, of course. Typically, once a roller coaster is launched, certain forces keep it moving, like:
Gravity
Inertia
Centripetal forces
Hills, turns, corkscrews and other features use physical forces applied to a roller coaster to send riders on an exciting journey. Natural forces on a roller coaster help slow it down to build riders' tension, then send them gliding down the tracks again. In those cases, roller coasters also use potential and kinetic energy to keep moving. Potential energy builds up as a car climbs the track, and it releases kinetic energy as it glides down hills. All of these forces on a roller coaster mean that the train itself doesn't need an engine to run.
Engineers must harness these forces in safe ways. A roller coaster will maintain speed or accelerate down hills and around curves depending on its design. Going up hills decreases the speed, making for fluctuating acceleration. These fluctuations can make a ride both exciting and safer since the car shouldn't pass certain speeds while heading into turns or hills. In those cases, the design is crucial, but so are particular parts of a roller coaster. Certain parts of a roller coaster function to keep riders safe on their journey.