How fast does an air-bubble travel in fresh or salt water?

Answer 1

I love this question! It's one of those, "Well, obviously...Um... what?" questions. So let's break it down: We'll assume the bubble is hollow (it isn't, but might as well be given the difference between air and water densities). Archimedes' Principle says that it will be buoyed up by the weight of the water that it displaces. This means that the upward force exerted on this bubble will be the same as the downward force that would be exerted if the bubble contained water. So the bubble gets accelerated towards the surface of the water at the same rate as a rain drop of the same volume would fall to the water from the sky (gravity applying to both). Ever notice in those undersea Scuba films that the bubbles hover near the mask before taking off? That's acceleration at work. So when you ask, "How fast?" I have to ask in return "When?" Because the bubble will be moving faster the farther it travels upward.

Since salt water is denser than fresh water, the bubble will displace more weight, but that's irrelevant, because it's still accelerated by gravity. A ship sitting in fresh water will sit lower than it will in salt water, but that's about weight displacement, not bubbles. Bubbles will rise at the same rate in either medium, if we set aside things like viscosity.


Great question.