[Cluts]

John Fleming  asked the Naked Scientists:
   
I have two balloons. One is filled with a heavy gas and the other with a light gas. I put them each in different rooms filled with the gas of the other. What types of factors determine which of the two balloons will win a race, where as the heavy balloon is released from the top of its room and the light balloon is released from the bottom of the other.

What do you think?

[Atomic-S]

The motion of the balloons will be affected by the following forces: First and most obviously, the difference in density between the two gases. Were that the only consideration, we would expect the two balloons to "fall" the same (magnitude of) distance in the same time. However, in addition to this, the balloon rubber itself has weight, and this weight does not reverse direction when the gases are interchanged. Also, the gas inside the balloons will be under pressure different than the gas outside due to the tension in the rubber. Therefore it will matter whether the compressibilities of the two gases are the same or not. Also, as a balloon moves, it is retarded by the viscosity of the external gas. If the two gases do not have identical viscosities, this will result in unequal performance.