Essentially my question is this: if I get a straw and put it in water, I cover my thumb on the top and pull out, water doesn’t want to leave. Idk why or how but regardless.
How come this same thing doesn’t apply for a swimming pool? Like a body of water is suspended because it has a lid or roof?
In: Physics
What matters more for the straw isn’t the roof, but the sides.
The reason the straw holds water when one side is covered is because air can’t get in. It can’t get in the sides because of the plastic, it can’t get in the top because of your thumb, and it can’t get in the bottom because of the water.
So the water “wants” to fall, but it can’t, because when it tries to fall, it is basically creating a vacuum behind it, and water in a straw isn’t heavy enough to beat the vacuum.
You see a smaller version of this whenever you pour water out of, say, a big water bottle. There, the water is heavy enough and the opening on the bottle wide enough to pull against the vacuum and let air slip past. That’s why the water comes out with that distinctive “glug, glug, glug” noice, because it’s letting in a bit of air, which let’s the water temporarily fall more easily. (And you can test this by poking a hole in the top of the water bottle to let air in from the top. Make a big hole, and the water will flow smoothly out the bottom with no noise or sputtering.)
And that’s also why it doesn’t work with bigger bodies of water. Eventually the size of the opening and the weight of the water is so high that the vacuum isn’t enough to stop all movement, even if you do seal the sides.
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