Well I noticed something with a helium balloon which a little weight was tied to its rope to avoid it flying away. When the weight is on the ground and the rope is taut, I pulled it down a little and let it go. It flew up and when the rope was taut again, it kept on going higher with a negative acceleration, traveled some height and then stopped and fell down again. Then I pulled it down much more than the last time, expecting it to gain a greater velocity due to buoyancy which is bigger than its weight, when the rope becomes taut, and travel a longer height, but what happened was it traveled a much smaller distance. Any explanation??