#### What role do gravity and inertia play in making a roller coaster work? — B

Gravity provides the energy source for a roller coaster and inertia is what keeps the roller coaster moving when the track is level or uphill. Once the roller coaster is at the top of the first hill and detaches from the lifting chain, the only energy it has is gravitational potential energy (and a little kinetic energy—the energy of motion). But once it begins to roll down the hill, its gravitational potential energy diminishes and its kinetic energy increases. Since kinetic energy is related to speed, they both increase together.

At the bottom of the first hill, the roller coaster has very little gravitational potential energy left, but it does have lots of kinetic energy. The roller coaster also keeps moving, despite the absence of gravitational potential energy. You can view its continued forward motion as either the result of having lots of kinetic energy or a consequence of having inertia. Inertia is a feature of everything in our universe—a tendency of all objects to keep doing what they’re doing. If an object is stationary, it tends to remain station. If an object was moving forward at a certain speed, it tends to keep moving forward at a certain speed. Inertia tends to keep the roller coaster moving forward along the track at a certain speed, even when nothing is pushing on the roller coaster. While the roller coaster will slow down as it rises up the next hill, its inertia keeps it moving forward.