system
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human
Buoyancy of vacuum?

So, a balloon filled with Helium flies because the gas inside the balloon is lighter than the gas in the atmosphere, correct? This also works when air is heated up. It's lighter, because it is less dense. 

Let's assume I could build a sphere, light as the rubber hull from a balloon but as stiff that I can create a vacuum in it. Would it fly extremely fast upwards? 

Thank you for your answers :-)
gpt
The buoyant force is equal to the weight of the displaced fluid, and the net force is the weight of the displaced fluid minus the weight of what you have replaced it with. The density of helium at sea level is 0.18 kg/m^3, or about 15% of the density of air. So a hard vacuum will still only experience a small amount greater of buoyant force as compared to the helium balloon. 

When you release the balloon, air resistance is preventing it from accelerating upwards ever faster, so it will only ever rise around sqrt(1.15) (7%)  faster than the helium balloon, neglecting weight of the balloon itself.