If you’ve ever been in the ocean in the vicinity of large breaking waves and have been unfortunate enough to get steamrolled by a wall of whitewater, you may have noticed how much more difficult it is to get back up to the surface through the whitewater compared to smooth water. Why is this? While it may be due in part to the difficulty in “gripping” the aerated water (to pull yourself to the surface you have to apply a force downward against the water such that it pushes upward on you), it also has to do with a reduction in your buoyancy, due to the lower density of the whitewater.
The demonstrator in this video may lack stage presence, but the demonstration gets the point across pretty clearly. When you inject air bubbles into a fluid such as water, the mixture or air and water will have a reduced density compared to the density of water. The buoyancy of an object in a fluid depends on the density of the fluid. If the object has a lower density than the fluid, it will float; if less, it will sink. In the demo, the boat has a density just barely less than that of water. You can tell because it floats very low. When the professor blows air into the water, he reduces the density of the fluid below that of the boat, and it sinks to the bottom. As soon as the air bubbles disappear from the water, the boat floats back to the top.
(Some of you may recall Archimedes’ principle: The buoyant force acting on an object is equal to the weight of the fluid that the object displaces. Using Archimedes’ principle, you can prove explicitly that objects will sink in a fluid less dense than they are, and float in one more dense.)
So next time you’re surfing at Malibu and get trammeled by a wall of whitewater, if you’re experiencing some difficulty clawing your way to the surface, just hold your breath and wait a moment. When the water smoothes out — and its density increases — it’s going to be a lot easier to get back to the surface for a gulp of air.
Adam Weiner is the author of Don’t Try This at Home! The Physics of Hollywood Movies.