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So this morning I purchased a coffee. As a not uncommon event the lid on the coffee cup didn't have a hole completely punched through (a gas inflow hole). This makes it difficult to drink the coffee because I'm essentially creating a vacuum when I suck out the coffee.

I was wondering then. Is it atmospheric pressure or gravity that causes the coffee to drain from the drinking hole?

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    $\begingroup$ More of a physics question, this, but it brings up some concepts pertinent to physical chemistry. $\endgroup$ – Buck Thorn Aug 8 at 4:31
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A combination of gravity and the suction you apply with your lips ("suck") help empty the cup, assuming as you do that the coffee in the cup leaves a vacuum as it exits. This is similar to the "problem" of emptying a bottle of liquid when you invert it (the mouth facing downward). The challenge is most evident with honey or ketchup, viscous liquids or those with higher surface tension that are more challenging to pour. Atmospheric pressure works against drainage of the container because there is no force (gas pressure) against the liquid at the inside top of the container. As liquid pours (or drips) from the mouth, bubbles of air travel up into the container, reducing the vacuum and allowing more liquid to flow out. Notice how thin PET water bottles crinkle when you force liquid out by suction? That's atmospheric pressure. Eventually you have to suck pretty hard to get additional liquid out, and can even make the bottle hang from your lips.

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Both, since atmospheric pressure does not exist without gravity. Atmospheric pressure is essentially the weight of air above something, which happens because of gravity.

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    $\begingroup$ Ultimately true, but not useful in understanding or explaining the immediate issue which is all to do with air pressure. $\endgroup$ – matt_black Aug 9 at 10:15

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