How is mechanical energy transformed and conserved in a roller coaster?

A roller coaster has ups and downs. When the cart is sitting at the top of the first big hill, is has lots of potential energy. As it descends the slope, it has some PE and some kinetic energy. At the very bottom, just before the cart starts to go up the next hill, it has almost entirely kinetic energy. This is the way mechanical energy is transformed on a roller coaster, but keep in mind also that some energy is lost in the process, due to friction on the rails, air resistance, and sound.