[Einstein Uae]

السؤال الأول :

Buoyant force = weight of displaced air
= density of air × volume × g
= ρaVg .
Weight of balloon = density of helium × volume × g
= ρhVg .
Downward force required = ρaVg - ρhVg = (ρa - ρh)Vg
= 10.8 N.