This relates to our recent themes of solid/liquid/gas and pressure, but mostly it was for fun. I brought a steel tube just a bit larger than a tennis ball, a 16-oz plastic soda bottle, a tennis ball, and some liquid nitrogen. After the usual LN2 demos, we went outside to make the cannon. I found a large object to hold the tube upright, then I loaded the bottle with about 4 oz of LN2, screwed the cap on tightly, dropped the bottle into the tube, and dropped the tennis ball on top. Then we waited for the liquid nitrogen to boil into gas, which would make the pressure inside the bottle roughly 100 times atmospheric pressure. It took about 5 minutes, and then the bottle exploded, propelling the ball hundreds of feet into the air and out into the neighboring field. Even the ripped-apart bottle shot up in the air, about 50 feet. It was awesome!
I had a second bottle and ball, but I was too greedy. I put in about 8 oz of LN2 to get a bigger boom, but after a 7-8 minute wait we heard a hissing. The bottle had a nonexplosive leak. Since it was starting to rain as well, we went inside, figuring it would not blow. But after another 7-8 minutes, we heard it blow. Bottom line: it's awesome with just 4 oz.
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