20. 밑줄 친 부분에 들어갈 말로 가장 적절한 것을 고르시오.

As a roller coaster climbs the first lift hill of its track, it is building potential energy—the higher it gets above the earth, the stronger the pull of gravity will be. When the coaster crests the lift hill and begins its descent, its potential energy becomes kinetic energy, or the energy of movement. A common misperception is that a coaster loses energy along the track. An important law of physics, however, called the law of conservation of energy, is that energy can never be created nor destroyed. It simply changes from one form to another. Whenever a track rises back uphill, the cars’ momentum—their kinetic energy—will carry them upward, which builds potential energy, and roller coasters repeatedly convert potential energy to kinetic energy and back again. At the end of a ride, coaster cars are slowed down by brake mechanisms that create __________ between two surfaces. This motion makes them hot, meaning kinetic energy is changed to heat energy during braking. Riders may mistakenly think coasters lose energy at the end of the track, but the energy just changes to and from different forms.

• 1
   gravity
• 2
   friction
• 3
   vacuum
• 4
   acceleration