Helium balloon raises up slowly through air .why?
(Question posted by Siju)
Helium is lighter than air and therefore the buoyant force (the upward force exerted by air on the balloon) will be more than the weight of the He filled balloon. So it rises up.
As it begins to move upwards, frictional forces (viscous force) also come into play which increase with increase in speed and soon the net downward force becomes equal to the downward force and thereafter the balloon rises up with a constant velocity.