Water displacement and gravity make strange bedfellows. Put something in a bathtub full of water, for example, and gravity pulls that something down, while water displacement pushes the water up. That would make for a pretty boring experiment, though, so we at Demo Science have concocted a better, more interesting way to show students (or whoever wants to pay attention) how gravity and water displacement work together to seemingly defy physics.
Build A Miniature, Boring Science Theme Park
For this Demo Science science demo, you’ll need two identical drinking glasses, a ruler (the sturdier the better), a pencil, some water, and a finger. One of the ones on your hand, not a severed finger. That would be gross.
Put your pencil down on a flat surface, like a desk. Put the ruler on top of that, with the pencil roughly in the middle. Fill both glasses about three-quarters full with water, then place one on each end of the ruler. Adjust the whole contraption so that the glasses almost balance, but not quite. It would probably be impossible to perfectly balance such a thing, so you’re dodging a bullet there in that you don’t even have to try. One end of the ruler should be lifted in the air, while the other rests on the table.
Now, it’s hypothesis time, kids! Tell the smelly little buggers what they think would happen if you added a goldfish to the raised glass. If they say, “that end would sink and the other would raise,” they’re correct. Ask them why; if they say, “because that glass would then be heavier,” they’re correct again. You must be a good teacher. Good on ya. But, if they get either of those wrong, give them all detention for a month. Just kidding: three weeks is probably enough.
Then, a tougher query: What would happen if you placed your fingertip in the water of the raised glass without actually touching the glass?
Let the kids discuss and theorize for a bit, and present their final hypothesis. Then, on to the sciencing.
Stick your finger in the water of the raised glass. Remember to not touch the glass itself!
Liquid Science (not to be confused with the Science of Liquids)
When your finger goes into the water, the balance will change and the rudimentary seesaw you’ve constructed will tip. While you’re not applying downward pressure on the water, you are displacing it in the exact volume of the portion of your finger that’s in the water. (The deeper your finger goes, the more water is displaced.) The weight of the glass increases by that of the volume of water that your finger displaced.
Does that make sense? No? Well, just trust me here. That’s pretty much how it goes. It’s science.