|Isn't there "some" acceleration at the very start and very end of an elevator ride? Why does one's stomach take a flop when the elevator stops and not when it starts?
Yes, there is acceleration at the start and stop of an elevator ride. As the car starts, it accelerates toward the destination and as the car starts, it accelerates in the opposite direction. Your stomach takes a flop whenever you feel particularly light, as when you are falling or otherwise accelerating downward. As you accelerate downward, your body doesn't have to support your stomach as much as normal and you feel strange. In fact, you feel somewhat weightless. You have this feeling whenever the elevator starts to move downward (and therefore accelerates downward) or stops moving upward (and there accelerates downward).