Thought Experiments | The Straight Dope | Salt Lake City Weekly

Thought Experiments 

How many sponges to absorb the Pacific Ocean?

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I've always wanted to go to Hawaii, but since I can't afford a plane ticket, I'm planning on walking there. How many 3-inch-by-4-1/2-inch sponges will I need to pack to absorb the Pacific Ocean? —Dave F., Springfield, Mass.

Now, Dave. Surely you realize this isn't a practical plan. Let's think about it a bit:

1. You can't isolate the Pacific from the other oceans of the world—you'd have to soak up most of the seawater on Earth. But OK, we'll pretend you could strategically pile the saturated sponges so they blocked water from running into the Pacific from the Atlantic, Indian, Southern and Arctic oceans.

2. The Pacific Ocean has a total water volume of around 158,000,000 cubic miles. This may not be an easy quantity to grasp, so let's express it in financial terms. A good surgical sponge made of polyvinyl alcohol can absorb about 14 times its own weight in water. A sponge of the size you specify can therefore absorb about 7.2 fluid ounces of water. To soak up the entire Pacific, you'll need about 3.1 sextillion sponges, costing maybe $8 sextillion. Perhaps you can negotiate a volume discount.

3. But hang on. Trying to stack up all those sponges will compress them to the point where more than 99 percent won't be able to absorb any water, defeating your purpose.

4. Fear not. You've now cornered the world market on surgical sponges. Sell a few, buy yourself a plane ticket. Better yet, buy yourself a plane.

I'm an English teacher and have a student who wants to know if a hairless cat will grow hair if you put Rogaine on it. We assume it's not safe, so we won't try it, but we are still curious to know if Rogaine works for our feline friends as well as people. —Paige Pittman, Indianapolis

Trying to grow hair on your own Mr. Bigglesworth is foolish. For one thing, people pay top dollar for Sphynx cats and other "hairless" breeds precisely because of their odd appearance—it'd be like buying a vintage convertible and welding a hardtop onto it.

Sphynx cats aren't actually hairless, but rather have fine hairs that fall out early in the growing cycle. In contrast to humans, where male pattern baldness results from scalp follicles gradually going dormant, Sphynx cats are from birth genetically incapable of growing more substantial hair.

Some might posit otherwise: The active ingredient in Rogaine is minoxidil, originally an anti-hypertension drug that relaxes blood vessels and would surely have vanished into the dustbin of medical history if more than 80 percent of patients taking it hadn't started regrowing scalp hair. Exactly why this happens is unclear, but because it appears to increase blood flow to the scalp, minoxidil may encourage hairs to enter their growing cycles faster. Since Sphynx cats have some hair, just exceptionally fine, couldn't regular application of Rogaine goose up their hair-growing cycle and make them, if not truly hairy, at least hairier than they are?

It'd be cruel to try to find out. Minoxidil can be used on some animals with care, and has been used to regrow hair on cats under veterinary supervision. But there's significant risk—we ran across vet reports of cats that died after their owners simply went ahead and applied minoxidil to hairless patches. Given the danger and the likelihood that minoxidil-induced hair growth on a Sphynx cat will be minimal to nil, my advice, teacher, is to leave those kits alone.

How fast do you have to be traveling to pass unharmed through moving helicopter blades? Is it even possible? —Mike Nielsen

So, Mike. This is a thought experiment, right?

Let's take a common helicopter, such as an Army UH-60 Black Hawk. ("Take" is to be understood figuratively.) The Black Hawk is equipped with a four-blade rotor that spins at 258 revolutions per minute and describes a 53.7-foot circle. To simplify the math, we'll assume you're 6 feet tall, 2 feet wide, and 1 foot thick, moving straight down toward the helicopter's blades feet first.

Assuming you were able to time your descent so you'd enter the plane of the rotor just as one of the blades had passed—and let's all say it: you can't—then if you were heading toward the very end of the blades, about 26 feet out from the center of the hub, you'd have to be traveling at 72 miles per hour to get through unscathed.

OK, you say, but what if I don't time it just right and the oncoming blade is a lot closer than I meant it to be—what's a safe speed then? In that case, if you're not going at better than maybe 5,900 miles an hour, I don't think you'll like how this thought experiment works out.

Send questions to Cecil via or write him c/o Chicago Reader, 350 N. Orleans, Chicago 60654.

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