When a helium balloon rises from rest, it gains Ep, at the same time, it gains Ek. So where does these energy come from? My guess would be due to density differential, the balloon will rise and this will displace the air molecules which will fall to take the place of the balloon. This reduction of Ep of air molecules is converted to Ek, which pushes the balloon, causing work done on the balloon, thus giving it energy. What do you think? Thanks.