When you block one end of a straw why does the liquid stay in place/in the straw?
In: Physics
Also, How wide could a straw be and have this still occur?
sometimes a question helps answer a question. Why do you think the liquid would stay in the straw? Liquids in gravity tend to drop towards the gravity. after all, if you flip the straw over, in your scenario, the water does not stay in place in the straw, but falls down and hits your finger.
The air around the straw has pressure which pushes on everything. When you put your finger over the top of the straw it prevents air from getting in there and pushing on the top of the column of water. When you lift the straw out of the water it is submerged in the water in the straw begins to lower which increases the space between your finger and the top of the water. This increase in space causes the pressure at the top of the straw to drop. The pressure at the bottom of the straw is maintained by the atmosphere which pushes up on the water holding it in the straw. The amount the pressure drops at the top of the straw is equal to the water pressure at the bottom of the cup you pulled it out of.
Simply because the forces on the liquid are at equilibrium. The weight of the fluid being pulled by gravity is equal to the force pushing up from the air pressure on the only open end of the straw (the bottom end).
When you release your finger, the pressures on both open ends is equal but you’re still left with gravity pulling the water down – so it leaves the straw.
I think the easiest way to think about this is with this question: “What would be in the straw if the liquid ran out and how would it get in?”
It would need to trade places with the liquid and there isn’t enough space for it to do so at the same time, thus no movement of any kind.
The liquid can not go out unless something else goes in to replace it. Normally, that would be air – but if you block the other end, then you’re blocking the air from getting in at that end, and the water itself is blocking the air from getting in at the other end.
A lot of responses are talking about creating a vacuum, which isn’t wrong, but sort of has it backwards. Vacuums don’t suck. Air pressure pushes. And if there’s a vacuum, well, there’s nothing pushing back to the air pressure pushes into that space.
When you have a straw, the air pressure from outside and below the straw is pushing up on the liquid. If you’re blocking the top of the straw, there is no equivalent force pushing down, and so the upward force from the air counteracts gravity.
If you release the top of the straw, now air is pushing in from the bottom, but it’s also pushing in from the top. Now gravity is pulling down, the air is pushing up and the air is pushing down, so the forces from the air pressure cancel each other out. Just leaving gravity to pull the liquid down and out of the straw.
There are three main forces at work here.
The first is a force called capillary action. In short, this is the tendency for a cohesive fluid (cohesive meaning it’s sticky to itself and holds together, like water) to cling to the walls of the inside of a tube and generate an upwards force. This upward force combined with the waters surface tension, another product of waters cohesive properties, allow the fluid to stay intact inside of the straw instead of just running off and separating like oil would.
The second is gravity, which obviously pulls the fluid towards the ground
the third is suction, which causes the air pressure in the straw to drop as the water is pulled away by gravity. after a short drop, the pressure is low enough to provide a suction force powerful enough to suspend the fluid.
so basically, capillary action and cohesion keep the fluid together, and gravity combined with suction keep the fluid in place by pulling in opposite directions.
If the liquid fell out of the straw, that could create vacuum unless the vacuum could be filled by something else. But air can’t get in from the bottom, because the liquid is in the way, and it can’t get in from the top, since your finger is in the way.
So instead, the liquid of the straw is held up by air pressure at the bottom pushing upwards and a combination of weight and the normal force from your finger pushing downwards.
Vacuum
For a cooler display of how strong the vacuum is, fill a glass completely to the top with water then take a piece of cardboard and hold it on the top of the water filled glass
Hold it in place as you flip the glass upside down, then take your hand away from the cardboard, if done correctly the vacuum in the glass will be strong enough to hold the cardboard onto the glass and hold the water in
It sounds like it shouldnt work and the water will just fall out, but give it a go (over a sink just incase it fails) its a cool little trick
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