To solve the rollercoaster problem, apply the principle of conservation of energy, which states that the total mechanical energy remains constant in the absence of friction. Initially, the rollercoaster has potential energy at the height of 100m, which converts to kinetic energy as it falls. To find the height when the rollercoaster is traveling at 20m/s, calculate the kinetic energy at that speed and subtract it from the initial potential energy to determine the remaining potential energy. This remaining energy can then be used to find the new height above the ground. Understanding these energy conversions is crucial for solving similar physics problems effectively.