When you block one end of a straw why does the liquid stay in place/in the straw?
In: Physics
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.
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