Answer this: Why do some things float and others sink?
Anyone who answers with something relating to ducks gets cement shoes.
Some of you might know that floating and sinking has less to do with weight or mass and more to do with density. This is... too true! That's it. Things less dense than water float, and things more dense than water sink. And we're done.
But we can think this through a little more with the next obvious question: Why does anything float in the first place? What force is acting on my floaties to help keep me above water? Is there some special additional force for things in water [or, more generally, liquids [or, more generally, fluids]]?
Nope! Read my lips: No new forces!
It may seem surprising at first to think that the same thing that holds up your floaties is holding you up right now [unless you're reading this while sky-diving in which case HOLY-BATMAN-AWESOME]. You don't fall through your chair/floor/ground into the center of the earth because of the electro-magnetic force. I know, boring. All the little electron clouds in your butt/feet push on the electron clouds of the chair/ground and repel, thus holding you up - blah blah blah.
The same happens in water. Your electron clouds and the electron clouds in the water repel, which is why we don't become one with the water upon entering it.
Yet, this is so unsatisfying.
It still doesn't get to the meat of the issue. Sometimes water can muster up enough strength to keep things afloat, and other times it just drops the ball.
The missing key is gravity. Since water can slosh around [unlike my chair, presumably] gravity is going to be busy keeping it in check, keeping it as low and flat as it can be [waves notwithstanding]. But gravity wants to pull the giant boat underwater too yet to do so requires pushing the water up higher. So only one thing gets to go down and fill up that volume. If the boat is more dense than the water, then it sinks since it is easier for the water to go up, against gravity, than the boat. And vice-versa. If the water in the volume that would be occupied by the boat weighs more than the whole boat, then water occupies that volume and the boat floats. In fact: <major surprising fact of the lesson> the weight of the water of the space that the boat takes up is exactly equal to the boat itself. [Whoa.]
Of course, how much floating action happens depends on just how different the densities are. As the density of an object approaches that of water, more and more of it sinks. Once it is greater than that of water, it sinks straight to the bottom [I hope we're all thinking of DiCaprio sinking in the Titanic. Or just the Titanic sinking, that works too.].
That's buoyancy.
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