I use cryogens in chemistry demos, and quick safe easy examples with zero clean-up are always welcome. I'd found about this one - the spinning ping-pong ball - through YouTube. The video has had over 2 million views, but I was underwhelmed - a slowly rotating and gently steaming ping-pong ball is hardly the kind of thing to impress an audience of schoolchildren. I finally tried it, reasoning that one pinhole ought to be better than two, and it vastly exceeded my expectations - it leapt off the table, hissing and steaming and spinning, and motored through 100 kids, clearing a path through them like the parting of the Red Sea. The video below is, sadly, not of that event, but a close-up repeat performance at home afterwards (note: the bare feet are NOT recommended! Wear appropriate clothing, including closed footwear).
Tell the audience only that you're immersing a ping-pong ball in liquid nitrogen. Ask them to predict what will happen (a good guess would be that it will collapse in on itself). The unexpected result will give them a good opportunity to apply some bits of knowledge they possess (depending on their age: that liquid nitrogen is very cold, and at its boiling point; that a gas has a much greater volume than a liquid; how a jet works/conservation of momentum; that water droplets are visible but water vapor is not) and to analyze the problem (they should be able to figure out that the ball has a hole in it, and that the hole has particular characteristics). You can guide them to the solution by being as vague or explicit as you need to be based on how much time you have, but I recommend allowing them to closely inspect the ball only once they have deduced the existence of the hole. Temporize by repeating the demonstration.
Disclaimer: I have no idea what rate the ball is spinning at, and 10,000 rpm is certainly gross exaggeration, but we are definitely talking some serious rotational velocity. It speeds up as the liquid nitrogen inside is consumed.
Tell the audience only that you're immersing a ping-pong ball in liquid nitrogen. Ask them to predict what will happen (a good guess would be that it will collapse in on itself). The unexpected result will give them a good opportunity to apply some bits of knowledge they possess (depending on their age: that liquid nitrogen is very cold, and at its boiling point; that a gas has a much greater volume than a liquid; how a jet works/conservation of momentum; that water droplets are visible but water vapor is not) and to analyze the problem (they should be able to figure out that the ball has a hole in it, and that the hole has particular characteristics). You can guide them to the solution by being as vague or explicit as you need to be based on how much time you have, but I recommend allowing them to closely inspect the ball only once they have deduced the existence of the hole. Temporize by repeating the demonstration.
Disclaimer: I have no idea what rate the ball is spinning at, and 10,000 rpm is certainly gross exaggeration, but we are definitely talking some serious rotational velocity. It speeds up as the liquid nitrogen inside is consumed.
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ping-pong ball ~ pin ~ marker pen ~ tongs ~ liquid nitrogen
Liquid nitrogen is at its boiling point of -196ºC. It's dangerous, but only on prolonged contact with skin (causes frostbite) or if confined (it will explode its way out of the vessel). Handle it with respect and in the right containers (stainless steel dewars), and wear appropriate clothing. Getting splashed with liquid nitrogen is not a problem because you are protected by the Leidenfrost effect. Getting more than splashed can cause serious burns. Companies like Praxair and Airgas sell it (you'll need an appropriate vessel), and universities always have a lot on hand in science departments.
ping-pong ball ~ pin ~ marker pen ~ tongs ~ liquid nitrogen
Liquid nitrogen is at its boiling point of -196ºC. It's dangerous, but only on prolonged contact with skin (causes frostbite) or if confined (it will explode its way out of the vessel). Handle it with respect and in the right containers (stainless steel dewars), and wear appropriate clothing. Getting splashed with liquid nitrogen is not a problem because you are protected by the Leidenfrost effect. Getting more than splashed can cause serious burns. Companies like Praxair and Airgas sell it (you'll need an appropriate vessel), and universities always have a lot on hand in science departments.
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I found that it needed a breath of warm air, or a quick grab with the hand to kick it off.
Also.. One time it began to levitate and rose 2 feet up off the table. I have not yet ben able to repeat this result. Anyone else have this happen?
Magnus effect:http://www.instructables.com/id/The-50-cent-Toy-Hovercraft-Helicopter-Demonstra/
Maybe you were thinking of the Wood Frog?
http://www.youtube.com/watch?v=UvCdOXG2rPo
Some frogs can survive freezing, but with LN2? I don't think so.
F•••s: This guy gives none.
I remember decades ago seeing a concept drawing in Poplar Science of a giant rotating dimpled sphere being used as a flying crane.
http://www.todayifoundout.com/index.php/2010/12/why-golf-balls-have-dimples/