Would a hollow metal sphere with a vacuum inside float?

30 May 2010

Question

Would it be possible to make a hollow metallic sphere float if you suck out all of the air and create a vacuum inside?

Answer

We asked Dominic Ford from Naked Astronomy.

Dominic - Yes, it certainly would be. What's important is whether the average density of the sphere plus whatever is inside it is greater or less than the density of the water that it will be floating in. So for example, a ship floats because although a ship is made of steel and that's very heavy, it's got air in there as well and the air is much less dense than the water it's floating in, so a ship as a whole floats. Now if you take a sphere, the metal outside of the sphere will be much heavier than the water, but because it hasn't got anything in it, that doesn't contribute to the density. So its average density be quite low. So it will float. It will actually float better than if you filled it with hydrogen or helium which, although they are lighter than air, they still have some mass to them, more than the mass of the vacuum which is nothing at all.

Chris - Indeed. It's a good party question that, isn't it? Which is going to float more, a sealed barrel full of air, a sealed barrel full of hydrogen, or a barrel with a vacuum, and that most people will go for the hydrogen and actually, it's the vacuum that floats the best.

Dominic - Yes, of course. You don't see barrels filled with vacuums very often because it's so hard to suck air out of a barrel!

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