Why does a beer overlow when someone hits the top of a beer with the bottom of theirs?
In: Other
If you take a bottle of water and whack the top of it hard, the bottle will move down very quickly. However, it effectively takes a second for the water to realise this and to move down with the bottle. This is called inertia. The result is a small vacuum at the bottom of the bottle. If you do this just right, the water gets pulled down by the vacuum with such force that it blasts through the bottom of the bottle. This is called a water hammer, and is sometimes the reason you head noisy heating pipes tapping.
Now, let’s replace the water with beer (and lessen the force). The beer stays put for a second and a vacuum forms. Except nature abhors a vacuum. So, the carbon dioxide that’s dissolved in the beer very quickly comes out of the solution to fill the vacuum. Except now you have a bottle of beer with a large bubble of CO2 underneath. This gas quickly rises through the beer, stirring up more bubbles as it goes, until the froth erupts from the bottle.
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