Hacker News Reader

Comment by dmalvarado | Hacker News Reader

I know it's supposed to be basic, but it still blows my mind that those two things still weigh less than the water they displace...

View on HN

> weigh less than the water they displace...

Well, they weigh exactly the same as the water they displace. :)

But I know what you mean. I think the cause is that we intuitively underestimate the weight of water. Probably because we normally deal with it in small amounts only. But water is pretty heavy.

> But water is pretty heavy.

A 20 gallon fish tank, which is a pretty small fish tank (1'x2'x1.25' or 2.5 cubic feet), weights 167 pounds - so about as much as a person.

Aren't people 70 water by mass?

If you press a human into a fish tank sized box it will weigh about the same, yes.

Yeah. And you can approximate many common materials as weighing "close enough" to water, if you're doing back of the envelope calculations. Organisms in general, food, building materials. Oil is about 90% the density of water. Plastic and wood probably vary from 50% to 130%. Metal and rock are obviously heavier, so throw in a multiplier.

It's great because you can just convert grams to milliliters or cubic centimeters (water is 1g/mL) and fudge it as necessary.

Fun fact: thanks to a browser-addon, I read this as

>A 75,71 L fish tank [...] weights 75,75kg

:-)

Haha, cool, is it Everything Metric (Auto Unit Converter)?

https://addons.mozilla.org/es/firefox/addon/everything-metri...

In comparison, 2.5 cubic feet of steel weighs slightly more than 1200 lbs.

Or try to tip over a kid's paddling pool that's full. It's surprisingly hard.

People are surprisingly heavy too

Reply on news.ycombinator.com