Mechanical energy is the sum of kinetic and potential energy in a system. E_(mech)=K+U Mechanical energy is conserved so long as we ignore air resistance, friction, etc. When we don't ignore outside forces, such as those just mentioned, mechanical energy is not conserved. read more
Yet in the absence of external forces doing work, the total mechanical energy of the car is conserved. Conservation of energy on a roller coaster ride means that the total amount of mechanical energy is the same at every location along the track. read more