A long time ago, when, I was studying physics at an engineering college, I had gotten proficient at “B.S.”-ing math problems on exams.
I had nothing on Abbott and Costello:
I’d seen this clip a few years ago but had forgotten about it. Hat tip to Epic Win FTW for the reminder.
I’ve always wanted to do this!
Yes, you can float a cannonball in mercury.
Whether an object sinks or floats depends on the densities of the submerged object and the surrounding fluid (liquid or gas). If the object (in this case the cannonball) is denser than the fluid (mercury), it sinks. Otherwise, it floats.
In the rare case that the two densities are equal, the object is “neutrally buoyant.” That is, it just hovers without moving up or down. Think of a days-old helium balloon.
Mercury is very dense, even for metals. Its density is 13.6 times that of water. Compare that with other metals we think of as “heavy” — iron (7.9 times as dense as water), copper (9 times water), or lead (11.3 times water). So even a solid brick of lead would float in this dear professor’s pool of mercury.
I would love to travel to the Dead Sea, for the opportunity to float on this body of salt water. I imagine I’d be as happy as this guy:
Salt water is more dense than fresh water (due to the salt, duh). That difference in density (only about 3%!) is enough to cause humans to be much more buoyant in salt water than in fresh water. In fact, I hear that it’s even difficult to swim in the Dead Sea because your added buoyancy is actually pushing you out of the water!
We could do some really cool things if mercury weren’t so horribly toxic to people. You would literally float so high on the surface of the pool of mercury that viewed from the side it would look like you were lying on a piece of plywood. It might even be possible to stand on a vat of mercury. Imagine if you only were submerged up to your ankles, or perhaps your mid-calf? You’d float while standing! (Staying upright might be possible, but walking might not be as easy as you’d think. There would be no friction keeping your feet stationary. They’d move side to side, and the mercury would feel as if you were walking on very slippery ice.)
Don’t try this at home, kids. Mercury’s bad for you. So if you happen to bump into this professor someday, don’t ask to play in his pool of mercury.