Buoyancy for Dummies

Buoyancy principles

Freelancers: you gotta love 'em... well, no, you don't. We don't, except perhaps in view of just how amusing some of their bull-bleep turns out to be once you get past the flowery bits generated by all the J-school and Creative Writing grads. Once in a while, however, we run across some who claims to have a BS in science instead of a BA in liberal arts, and we regard their failure to deliver on technical questions with something approaching horror. That's what happened when one of our staffers ran across John Brennan fumbling the question, "Does a Balloon with Helium Rise Higher than One With Oxygen?" which he posted to SeattlePI.com's education section.

Brennan, believe it or not, got the answer right: because of helium's low atomic weight compared to that of O₂, a helium-filled balloon will rise until it reaches an altitude where it displaces a volume of air with mass equal to that of the helium (this presupposes that the balloon will not simply burst). The balloon filled with oxygen will do the same, except that oxygen is more dense than air, so the second balloon will sink.

Yeah, Brennan got it right, although he tried to carefully copy the densities of the three gases at STP, he neglected to transcribe the density of oxygen (1.331 kg/m³ at NTP), which wasn't helpful. Not only that, but either John blew it or he allowed his content editor (a pox on the scientific world) to attempt to explain the reason for buoyancy as "Think about a balloon floating in the air. The surrounding air is pushing the balloon upwards thanks to air pressure, while gravity is pulling it back down. If the upwards or buoyant force is greater than gravity, the balloon will rise, while if it is less the balloon will sink."

Well, no, John, the "surrounding air" isn't "pushing the balloon upwards thanks to air pressure." The helium-filled balloon, like a block of wood in your bathtub, "floats" in the air because it masses less than the volume of air it displaces. All that air pressure has to do with the process is that, as the balloon rises in the atmosphere, the mass of the displaced volume decreases until equilibrium is reached (we'll ignore the mass of the latex balloon and changes in temperature).

Half-credit, Mr. Brennan. If you had discussed "buoyant force," we'd probably have let it slide; but instead you tried to blame buoyancy on air pressure. You probably shouldn't have let some J-school graduate mangle the principles while attempting to simplify your explanation... but because you did allow it, you're our Dumbass of the Day.

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