Maybe I’m just too dense to figure this out….
In: Physics
The real answer has to do with density. Density is mass divided by volume. Helium is less dense than air. As in, a tub of air is heavier than the same tub of helium. Obviously helium is also much less dense than the rubber balloon.
When you inflate the balloon you are slightly increasing its mass and weight by adding helium, but at the same time you are decreasing its density, because it’s getting way bigger.
Now we can think about gravity. Gravity is pulling everything down, but more dense things get pulled down more strongly. So, the air around the inflated balloon gets pulled down more strongly than the balloon itself. So, the air goes down and pushes the balloon up.
This is what the other commenters are talking about when they say ‘displacement’.
Just to be extra clear, letting helium out of a balloon would make it lighter, in terms of both weight and mass.
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