Um, yeah energy is conserved. It's called the conservation of energy

A roller coaster is started and as it moves it has kinetic energy. As it rises to the top of a curve, it slows down as the energy is turned into potential energy. At the top of the track, it has the maximum potential energy because it's as high as it will go. Potential energy is measured relative to a zero point in height. As the roller coaster is pulled back down by gravity, the potential energy is converted back into kinetic moving energy and the coaster speeds up as it gets lower to the ground zero-point.

basically you have a little balance with kinetic energy hanging down on one side and potential energy on the other, strung over a pulley. When kinetic moves down, potential moves up.