Paul Asimow (PhD '97), Eleanor and John R. McMillan Professor of Geology and Geochemistry
Consider a He-filled balloon with a volume of 1 cubic meter. At 25°C and 1 atmosphere pressure, can you hold it without floating away?
What if you tied it to your cat?
What if you tied it to your hamster?
In general, how much mass can 1 cubic meter of He lift in air (the answer is a satisfyingly round number)?
BONUS: See if you can do this in your head, from memory, without looking up any constants or equations.