Real roller coasters lose energy to friction and air resistance, which limits the energy they have available to travel the rest of the track. Unbalanced forces cause the cart to accelerate according to Newton's second law, speeding up on the downhills and slowing down on the uphills.

A roller coaster loses its mechanical energy primarily through the high-fidelity actions of friction and air resistance (drag). As the coaster wheels roll along the steel or wooden track, "rolling friction" converts a portion of the train's kinetic energy into thermal energy (heat), which is dissipated into the track and the surrounding air. Simultaneously, as the train moves at high speeds, it must push through the air molecules in front of it; this air resistance acts as a counter-force that further bleeds speed from the system. These "non-conservative forces" are the reason why a coaster can never return to its original starting height without a second lift hill or launch—the system is constantly "leaking" energy into the environment. Engineers must use complex mathematical models to account for this energy loss to ensure the train has enough residual momentum to clear every loop and hill in the circuit safely.