In this Hila Science video on atmospheric pressure (https://www.youtube.com/watch?v=xJHJsA7bYGc&ab_channel=ScienceOnline) from 2:30-3:05, it shows the common demonstration of turning a glass of water over a card and the card remains due to atmospheric pressure on the card and a vacuum at the top. If there is air in the glass before the card is placed over the opening, why does a vacuum form? Shouldn't the original air that was in the glass above the water just move to the bottom of the glass after the glass is turned over? In this question (Why does the upside down water cup and card trick work?), the question was mostly about water pressure. My question is mostly about the air. And why would the air pressure be reduced inside the glass?
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Gas is compressible and is compressed by atmospheric pressure. When the glass is upside down the water "pulls" on the gas sucking it to lower pressure. Lower pressure than outside the glass = vacuum. But really don't discount the surface tension of water either. Think about why you can raise a half full glass upside down in a bathtub above the water level of the tub – R. Rankin Feb 07 '23 at 00:29
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Doesn't there have to be NO air in roder for there to be a vacuum? How is the water 'pulling' on the gas? There is no air in the gas. I'm confused by your comment. – suse Feb 08 '23 at 02:44
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"partial" vacuum is just a difference in pressure. The with the air in the top of the inverted glass, the water acts like a seal, while also it's weight is "hanging" on the air. That downward force of the water drops the pressure in the air pocket above it, until the two opposing forces cancel – R. Rankin Feb 08 '23 at 02:48
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you could picture replacing the water with a plunger hanging out of the cup. If you try to pull the plunger downward, (same as the weight of the water does) then you can feel the vacuum resisting your efforts, harder you pull, harder the vacuum (which is a technical term by the way lol) – R. Rankin Feb 08 '23 at 02:51