Friction is an important factor in the physics of roller coasters. The roller coaster loses energy due to friction and air resistance as it moves. The track and wheel bearings are the objects that cause friction. The sum of potential energy and kinetic energy would stay the same if there was no friction. With friction, the energy will constantly go down through the ride. At the end of the ride, the coaster won't be able to climb as high as it could at the start of the ride. So now you know that if the coaster is going very fast, then it has a lot of energy.
