# eli5 why can a straw carry water without the bottom dripping but not a pool?

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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

The difference is the surface area, and the effects of surface tension

In the small space at the end of a straw, the air pushing up on the water can’t “get past” the water due to surface tension, so the water stays suspended.

In a swimming pool, the surface tension wouldn’t be as strong, and the water would be able to “get by”, making the water fall out the bottom.

Air has weight and pushes on everything all the time you just don’t notice because it pushing everywhere evenly . Your finger on top of the straw keeps air from entering the straw from the top. The air at the bottom of the straw is pushing on the water in keeping it in the straw.

Water has surface tension. Trying to think of a good way to explain it, it’s almost like the water develops a “skin” that’s trying to hold itself together. It’s why water forms drops on a surface instead of just spreading out thinly, and why things can rest on the surface of water (pond skater insects etc.)

The hole in the bottom of a straw is small enough that surface tension can hold it together. But as the opening gets bigger surface tension isn’t enough and it breaks and the water falls out. That’s why you can’t do the same trick with a cup.

Also you need to hold the end as if the straw is open the cylinder of water can just slide out as a single piece. When you hold the end you create a vacuum that holds the top of the cylinder of water in place.

Vacuum pressure keeps the water in place in a straw because it is a very small amount and weighs very little.

A pool amount of water on the other hand is very heavy and weighs more than the how much the vacuum pressure can hold up.

you are thinking about this the wrong way, the water is not suspended because of a lid or a roof or in the example of the straw the finger.

all the finger does is prevent air from getting in from the top so now in order for the water to drop down air needs to come in from the bottom.

Because the straw is narrow and theres only very little water in it the surface tension of the water is strong enough to hold the water up and prevent air from getting in.

if you increase the surface area and mass of the water by using a very large straw the surface tension wont be strong enough to hold all the weight and everything will drop down.

Because of “surface tension.”

In order for water to fall, something needs to replace it or else you get a vacuum. With large amounts of water like a pool, there’s enough area for the surface tension to break and let the air and water swap places.

However, when you decrease the surface area of the water enough, such as in a thin straw, the water _wants_ to fall out, but can’t due to the fact that the surface tension won’t let air in to replace it.

Surface tension. When you cover the top of a straw the reason it doesn’t fall out is that there is a vacuum at the top of the straw between your finger and the water. The atmospheric pressure of the air surrounding the straw pushes the water up into the straw with an amount of force that balances the weight of the water. The straw is small enough that the surface tension of the water prevents the air from getting up into the straw through the water (which would get rid of the vacuum).

If you were to take a full pool and flip it upside down and lift it into the air all the water would fall out because the pool area is very large and surface tension is not a strong enough force to prevent air from traveling up the water. With no vacuum there is no way to keep all that water there.

They put a piece of paper to prevent air flow and the water doesn’t fall. If you could do that with a pool it would work. With a straw the surface tension is enough that you don’t need paper to prevent air flow.

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.

I fully understand how the water in the straw works, but what is OP trying to describe with the swimming pool?
Swimming pools have bottoms, not tops…

Air pressure and surface tension.

Surface tension helps to keep the water surface together, so it doesn’t bend or distort much on its own. Without surface tension, part of the water could start flowing out. Surface tension mostly acts to provide a surface for the other factor here, air pressure.

Air pressure pushes (effectively) equally on all surfaces of an object: top, bottom, and sides. If you block up the top of the straw, that means that the only place that can feel the atmospheric pressure is the small hole at the bottom of the straw. That pressure, in the right circumstances, can be greater than the weight of the water pushing down.

As a result, you can only make this work if:

1. You don’t have too much water to hold up (because the weight is too great and will overcome surface tension + air pressure)
2. The hole the water can fall through isn’t very big
3. The water is mostly stationary (no internal flow/currents)
4. You close up all holes that aren’t pointing more or less straight down.

It doesn’t work with swimming pool sized things because the water is too heavy, breaking point 1.