Re:Huge helium discovery 'a life-saving find'
by thaumasiotes on Tuesday June 28, 2016 @04:14PM
(#11997147)
While I appreciate the answer, it's not a great explanation of the balloon example, since a balloon starts out effectively containing nothing. The problem there is that as you add helium, the volume of the balloon increases.
I'm a little more comfortable with saying we do some weighings in a vacuum (say, to determine the density of air at a given pressure) than with saying we'll start with a cylinder that contains a vacuum. For example, our 40kg cylinder, when airtight and containing vacuum, will weigh less than a 40kg object should, and I think that muddles the example.
Re:Huge helium discovery 'a life-saving find'
by eru on Tuesday June 28, 2016 @08:03PM
(#11998554)
Only the difference between vacuum cylinder and helium cylinder matters. (Since the buoyancy in air only depends on the volume of the cylinder, not what's inside.)
I'm a little more comfortable with saying we do some weighings in a vacuum (say, to determine the density of air at a given pressure) than with saying we'll start with a cylinder that contains a vacuum. For example, our 40kg cylinder, when airtight and containing vacuum, will weigh less than a 40kg object should, and I think that muddles the example.