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Why Glass Bottles Shatter at the Bottom When You Smack the Top with Your Bare Hands

There’s nothing quite like using the scientific method to figure out what’s happening in a silly party trick that we’ve seen way too many times (if you’ve been to bad parties, at least): the ol’ smack the top of a glass bottle and watch the bottom burst out in shards gag.

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The always intrepid Mark Rober tested out multiple theories to find out what’s actually causing the bottom of the bottle to shatter and used a super high speed camera to offer up convincing video evidence.

The theories he tests are all about pressure. One is that air pressure is what’s causing the bottom to give out. Another suggests that non-carbonated liquid works better in shattering glass than carbonated liquid and that the liquid getting smacked down to the bottom is breaking it. Finally, the air outside is the bottle causing it to break by pushing in on the bottom of the glass. It’s interesting to find out why, if only to see multiple glass bottles break.

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DISCUSSION

gatorbait28
gatorbait28

Water is an incompressible fluid. Smacking the top of the bottle pressurizes the air inside the bottle. That pressure pushes on the water, but as the water cannot compress, the force is transmitted through the bottle causing the bottom to give out. Simple physics.

This is why this will not work without water and also will not work with a completely full bottle. There must be some air in the bottle for this to work. Without water, the air just compresses. Without air, there is no air pressure to force the water down towards the bottom of the bottle.