Timeline for Why should water fall down from an upside down glass of water?
Current License: CC BY-SA 4.0
7 events
| when toggle format | what | by | license | comment | |
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| May 31, 2021 at 13:39 | comment | added | user14492 | It's because the vaccum inside the glass is stopping the water from falling. When there's low pressure in glass due to water being pulled by gravity which forces the outside higher pressure to push the card back up. "The atmospheric pressure against the card on the bottom provides enough force to push the card against the test tube" this is ambiguous; outside has no reason to push the card if there's no pressure difference inside. You can completely fill the glass with water and it would still work! | |
| May 29, 2021 at 5:58 | comment | added | DKNguyen | @PeterCordes Now that you mention it, if you plug up a syringe with your finger and pull it on the handle it really won't want to come out. Even though the friction in the syringe is more than enough to overcome gravity, you pull it on it much harder than gravity would. | |
| May 29, 2021 at 5:16 | comment | added | Peter Cordes | @DKNguyen: If you turn an ice-cube tray upside down, the ice doesn't fall out even if you shake it. Because it's frozen to the tray. If you made a plug of ice with an air-tight seal in a test tube, I'd certainly expect it to be held in by air pressure. The seal could be a layer of melt-water. (Assuming the ring of water is small enough for surface tension to stably seal against the weight of the ice plug). Obviously if you just put some loose ice cubes in a tube, you can dump them out easily, so the question is how tight a fit it is. An airtight metal plug would equally stay in place. | |
| May 28, 2021 at 7:00 | comment | added | silverrahul | My guess about ice vs water would be that, ice is more likely to have small gaps along walls, whereas water is more airtight. I dont think, the bill nye trick would have worked if it was ice instead of water. Unless, it was due to ice being stuck frozen, in which case obviously, no card would have been needed because air pressure would not be what is causing it to stick | |
| May 28, 2021 at 6:35 | comment | added | DKNguyen | @silverrahul Even if it worked the surface of the water is really malleable and imperfect compared to the card and I would imagine pressure variations and imperfections in the water surface would quickly cause concaves in the water which would just get worse and get behind some water, isolating making it fall away and it would just repeat until everything had fallen away. And supposing it did work, I don't see how it being water is any different than say, a block of ice which we know just falls out. | |
| May 28, 2021 at 6:22 | comment | added | silverrahul | I think the question is why does this not happen without a card . Isnt the pressure on the card same as the pressure on the surface of water ? | |
| May 28, 2021 at 5:21 | history | answered | lamplamp | CC BY-SA 4.0 |