Imagine you are in an elevator accelerating downwards with acceleration g, holding a balloon (which would fly upwards if left outside). Now if the thread you are holding snaps, what will happen to the balloon? Since the acceleration felt inside the elevator is 0, the balloon should not experience any buoyancy and continue to stay at the same position. The difficulty I am having in understanding is, if this experiment is observed from outside (i.e. from an inertial frame of reference), the balloon seems to be in a free fall towards the ground, where as it should actually be rising up. Where is the mistake?