re: What if I dropped a beer mug off the Empire St. Bldg.?
I can understand how maybe marbles might fall at the same rate as the glass. But would bouyancy have no effect on the bubbles in the beer? That’s what drives them upwards against gravity when the glass and beer are falling at the same rate on my desk (namely 0).
Why would bouyancy not be present when the glass and beer are falling at a rate greater than 0, but still the same?
Thanks
Buoyancy would not be present if the glass is in free fall; i.e., accelerating downwards under the influence of gravity. In this condition, the glass and beer are effectively weightless (think the famous “Vomit Comet” training runs), and so there is no “up” direction in which buoyancy would drive the bubbles.
Any other motion by the glass that is not complete free fall would mean that there is still some gravitational force felt by the mug. In particular, there is some discussion of air resistance slowing the mug down in the other thread related to this column.
I can’t resist. I accept that the bubbles in the beer would remain stationary while the whole package is free-falling in a vacuum.
I wonder, though - do the various bubble-producing points continue to produce bubbles? (Have you ever noticed that the bubbles in a calm mug of beer seem to erupt from just a few points in the mug?) Or do the bubbles now come from all over? In either case, the bubble volume would grow inside the mug, displacing beer.
This is troubling because in free-fall you would not be able to simply drink from the mug. If you forgot your straw, you could end up losing beer!
Okay, that makes lots of trouble. If it is in vacuum, then while the air resistance has been removed, you have a totally different problem - the beer want’s to flash sublimate. So you really quickly have an empty mug.
So now you need a mug of beer with a bubble top falling in a vacuum.
The “bubble producing spots” are called nucleation sites. It is the same thing as boiling water. The bubbles form at rough spots on the sides of the container, or other object in the liquid (like a spoon, or stir stick). I think the bubbles would form at the nucleation sites and continue to grow, and maybe merge if bumping other bubbles.