# Why an empty cup which you turn upside down doesn’t fill up with water when dunking it straight down into the water

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Why an empty cup which you turn upside down doesn’t fill up with water when dunking it straight down into the water

In: Physics

Air is lighter than water and wants to go up, it won’t go down because the water is heavier, so it’s trapped

The cup isn’t empty, it’s full of air. The water pushes ‘up’ on the air but it can’t escape the walls of the cup.

Because the air in the cup is taking up space. If there was no air or the air was pumped out, the water would fill the void.

Interestingly, the air is compressible., so at sea level, the air would keep the water out. If you lowered the cup 33 feet, the air would compress 50% allowing the cup to fill halfway with water.

This is because, at sea level, the ambient air pressure is about 15 psi. If you dive down just 33 feet, the combined air pressure and water pressure doubles to 30psi. Basically, the water at 30psi compresses the air that started at 15 psi to the ambient 30psi.

This all becomes very relevant if you’re trying to breathe underwater with SCUBA gear. At 33 feet it takes twice as much air and pressure to inflate your lungs. At 66 feet it takes 3 times as much and so forth. More importantly, when coming up you have to keep your airway open because the air will expand as you rise through the water. If you tried to hold your breath, you’d potentially cause lung damage. Most of basic SCUBA training is simply about not holding your breath…. ever.

There is lots of air in that cup. Air is lighter than water, so it stays floating on top of the water. It can’t go up because of the base of the cup, it can’t go to the sides because of the walls of the cup, and it can’t go down because there’s heavier water in the way.

It will appear to fill up with water if you push it down deep enough for the pressure to significantly compress the air in the cup.