“ Movie" most clearly describes the word "medium" because it's a means or a form through which people are communicated to.
The author describes what potential energy is and explains how roller coasters use potential energy as they rise to the top of the hill. She then explains kinetic energy, what it is, and how a roller coaster is using it when it comes down the hill. Later, Kibler described how the coaster will run out of kinetic energy. Even without brakes, she says, friction would cause it to eventually come to a stop. This all supports Kibler's argument that science can be learned from amusement park rides.
