[ELI5] why does plugging one end of a straw not let the water out of the other end when held up?
In: 4
Water is kind of “sticky” at small scales. With a small enough opening, the water won’t let any air through it to fill the space at the top of the straw that would be left empty if the water dropped. So a small vacuum is created that holds the water up.
Literal ELI5:
The water in the straw tries to start falling down, but as it starts to fall there’s nothing left above it to fill the gap where it was. There can’t be *nothing* there, so the water has to stay up in the straw until something else can go in that gap.
If you let go or poke or squeeze the straw so that some more air will be able to get in above the water, the water will be allowed to fall.
Most of the time on Earth there can’t be *nothing*, and there always has to be something like a air, water or other things. When there is a place with nothing we call it a vacuum. They exist when humans make them, like in a vacuum cleaner or other things that ‘suck’ things up. Away from Earth, except for planets and stars space is all a vacuum.
The average air pressure at sea level on earth is 1 atmosphere/atm. This pressure is constantly pushing on all things from all sides.
When you have a straw filled with water, the water falling out the bottom is constantly replaced by air coming in from the top due to this air pressure. If you plug the top, air cannot replace the falling water from the top anymore. The only way for air to get into the straw now is from the bottom… where the water is supposed to be coming out of. As air is more compressible than water, the weight of the water increases the volume of air inside the straw without increasing the mass/amount of air (like a plunger or syringe), decreasing the pressure inside the straw. As gasses want to fill spaces and the outside air pressure is 1 atm, air is pushing back up the straw, creating an equilibrium where the water cannot fall and the air cannot fill the straw.
Other things to note: this is really only possible with straws and other similarly-narrow tubes due to water cohesion (sticking to itself strongly). It’s hard to replicate this even with a 2-liter bottle, as the force of water cohesion is not enough to overcome the ambient air pressure from breaking the tension and forcing itself up into the bottle as bubbles. This is why tipping over a 2-liter causes liquid to come out slowly in bubbling bursts. Also, this couldn’t happen in space. With 0 air pressure, there’s no force pushing the water back up the straw, so it would just fall out.
the weight of the water is trying to fall out of the straw, the weight of all the air in the atmosphere above you is pushing to get into the straw, as long as the straw is less than about 10 meters tall the air pushes the water in harder than the weight of the water pushes out
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