As the roller coaster descends from a high place, the height decreases, and the speed increases, so the potential energy decreases, and the kinetic energy increases. Therefore, potential energy is converted into kinetic energy while the roller coaster is descending.
On the other hand, while the roller coaster is going up, the speed gradually slows down and the height increases, so the potential energy increases as the kinetic energy decreases. So, while the roller coaster is going up, kinetic energy is converted into potential energy.
Like this, the potential energy can convert into kinetic energy or kinetic energy can convert into potential energy. The sum of potential energy and kinetic energy of an object is called mechanical energy.
Law of conservation of mechanical energy
Consider the case of tossing a ball upwards or in a free fall. In the absence of air resistance or friction, as the potential energy decreases, the kinetic energy increases. Conversely, as the potential energy increases, the kinetic energy decreases accordingly. Therefore, the mechanical energy of an object is conserved constant.