# Background Essay: Designing a Roller Coaster

Through the years, roller coasters have gotten faster, longer, higher, and more serpentine. Despite the changes designers have made to heighten the thrill of their rides, roller coasters continue to rely on a few fundamental principles.

Roller coasters have no engine. Instead, they are pulled by a motor-driven chain to the top of the first hill, called the lift hill, and then released. From this point on, the roller coaster coasts, powered by gravity and inertia.

Inertia is the tendency for moving objects to maintain their forward velocity. It is also what allows a roller coaster, once released, to continue over hills and through loops in the track without receiving power from an outside source. At the top of the lift hill, the gravitational potential energy of the coaster is maximal. As the coaster moves down the hill, pulled by gravity, it accelerates, and potential energy is converted to kinetic energy. As the roller coaster travels up subsequent hills along its path, kinetic energy is converted back to potential energy.

Roller coaster designers must take into account the energy loss that inevitably occurs as a coaster moves along a track. Friction between the train and the track and between the train and the air resist the roller coaster's forward movement and cause a gradual loss of the energy that was originally stored as the roller coaster was towed up the lift hill. Because of this, engineers must make each successive hill smaller than the last so that the train will be able to get over each one.

Loops provide a more difficult challenge for roller coaster designers. Most importantly, the coaster must have enough speed to travel all the way through the loop without stopping. Originally, engineers designed round loops. However, just as a coaster loses kinetic energy, expressed as speed, as it travels up a hill, it also loses speed near the top of a loop. With a round loop, the coaster must enter the loop at a very high speed. Today, engineers make teardrop-shaped loops that tighten the curve near the top of the loop. This ensures that even at the top of the loop when the coaster is at its lowest speed, it still presses against the track, and the riders press against the coaster.