Have you noticed that many traditional rollercoasters slowly climb an enormous first hill before racing through a series of smaller hills, turns and drops? That first climb is doing something extremely important: it is transferring energy to the rollercoaster train. As the chain or lift system pulls the train upwards, energy is transferred into the train's gravitational potential energy store. The higher the train travels, the greater its gravitational potential energy relative to a chosen lower reference level. When the train passes over the top and begins travelling downhill, its gravitational potential energy decreases. At the same time, its kinetic energy increases, so the train accelerates. Near the bottom of the drop, much of the mechanical energy that was associated with height is now associated with motion. When the train climbs the next hill, some of the process reverses. Its kinetic energy decreases as gravitational potential energy increases, so the train usually slows down as it climbs. In an ideal rollercoaster with no air resistance, friction or other energy-dissipating effects, the total mechanical energy—the sum of kinetic and gravitational potential energy—would remain constant. A train could theoretically convert energy backwards and forwards between height and motion without losing mechanical energy. Real rollercoasters are different. Air resistance and interactions involving the wheels, bearings and track transfer some mechanical energy into thermal energy and sound. The energy has not disappeared: total energy is still conserved. However, less remains available as mechanical energy for the train's speed and height. This is why, on a traditional rollercoaster without another powered launch or lift, later hills are generally lower than the initial lift hill. The train needs enough kinetic energy to climb each hill while still maintaining a safe speed. Some modern rollercoasters overcome this limitation by adding powered launches or additional lift systems that transfer more energy to the train during the ride. Engineers calculate these energy transfers when designing the track. They must also consider forces, acceleration, air resistance, wheel rotation and the forces experienced by passengers. Computer simulations and physical testing help engineers make sure the train can complete the track safely under different conditions. The same principle appears throughout physics and engineering. Energy can move between stores and be transferred between objects, but it is not simply created or destroyed. So when you reach the top of that enormous first hill and look down at the track below, the rollercoaster has already received much of the energy that will carry it through the ride. The terrifying first drop is really an enormous energy transformation waiting to happen.