Been wondering about this for a while. Two absolutely identical (in every way) vehicles are traveling 100 miles, at a perfectly constant speed of 75mph, in a vacuum. The starting elevation is exactly 1,000' above sea-level. The finishing point is exactly 1,000' above sea-level. But.... Vehicle 1: encounters hills along the way. Vehicle 2: travels on perfectly flat terrain I'm curious how Mr. Newton's laws apply to these two cars. It seems the energy required would be identical, since one has to do work to climb the hill (resisting the acceleration of gravity), but will catch the "tailwind" of stored potential energy in the form of gravity on the other side of the hill. Thus, making everything exactly equal (in a vacuum anyway). Thoughts??