# 10,000 rpm ping-pong ball

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.

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## Step 1: Equipment

You'll need:
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.

## Step 2: Preparation and execution

Poke a hole into the ball with the pin at as shallow an angle as possible, and add random markings with the pen (so the rotation is visible). Pick up the ping-pong ball with tongs, and immerse in the liquid nitrogen for about 10 seconds. Remove and release. Throwing it on the floor works well - it will hiss and spin and move horizontally in a random direction. In the video, I confined it using an extension cord so it stayed in close camera range, but it's better if you don't - much more fun to have it go whizzing off somewhere.

When the ball is immersed in liquid nitrogen, the air inside is chilled and the pressure drops (according to PV = nRT, and further yet because the oxygen will condense). The low internal pressure sucks liquid nitrogen in through the hole. On removing the ball, the nitrogen inside begins to boil as it heats up, and because gaseous N2 takes up about 700 times the space of liquid N2, the gas has to go somewhere, and it rushes out of the hole and turns the ball into a jet. Because the hole is at an angle, the resulting force causes the ball to rotate (spin) rather than translate (move). The steam is water condensing out of the air due to the cold gas, and so this experiment is more impressive in humid conditions.
papercrafter4083 years ago
thats so cool! but... i personally think its is spinning at 9,997.643 RPM...
5 months ago

nah 9 999.999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999 RPM

eth1232 years ago
So sweet
great job man was it hard
Covo3 years ago
I have done this with my 8th graders... Very engaging!!! Your kids look (and Act) a bit younger (when my ball hit the ground and went towards the kids, they all went clamoring after it!)
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?
3 years ago
If it's spinning around its horizontal axis instead of its vertical axis, it could spin fast enough to create the "Magnus Effect"... making it "fly". Pretty cool either way!

Magnus effect:http://www.instructables.com/id/The-50-cent-Toy-Hovercraft-Helicopter-Demonstra/

makendo (author)  Covo3 years ago
Yeah, I think these kids were about 4th grade, and they had just seen me smash a couple of onions, and I don't think they thought I was particularly trustworthy. I've never found the ping-pong needed warming up, but certainly some worked a lot better than others (probably to do with the exact shape of the pinhole). Never seen one levitate, but I plan to try throwing a full one - I think the Magnus effect will kick in and we might see some neat effects (see conversation with biochemtronics, below).
3 years ago
You have aroused my curiousity: What happened when you smashed the onions to cause your students to become suspicious of your actions?
makendo (author)  stringstretcher3 years ago
Bits of frozen onion flew into the audience. No big deal - I told them not to try and pick the pieces up - but their teachers kind of overreacted.
stringstretcher3 years ago
Attach the ball to a string, and hang it on a flexible rod. It may fly like crazy then. I have to try all this!
mikenaly3 years ago
I had the wing thought too last night after posting my previous comment. They might be hard to shape properly, but if you could get them balanced well, it might pull off a pretty stable imitation of a helicopter. Make for some interesting additions to your school room demonstration if you can work out some of the details anyway. I also wondered about the possibility of using a larger ball. Something like one of the plastic baseballs? I realize you would need a larger container for the liquid nitrogen, but wouldn't the larger ball result in a greater volume of the nitrogen being pulled in, and therefore, a much longer, spin time? Also, I am not sure, but I suspect, the larger volume might allow you to go back to the implied two holes in the you-tube example you cited as somewhat underwhelming and still get an increase in rotational velocity. The potential drawback would, of course, be the increased weight of the larger ball causing greater drag though. Just a few random thoughts. I would experiment myself and post the results, but I just don't have the resources available right now.
makendo (author)  mikenaly3 years ago
Youtube has quite a few different versions of this experiment, and one of them was a larger ball - and it seemed to spin quite slowly. I do want to try throwing it, though, and see if I can get it to veer around in interesting ways. Not sure how I'd get the wings to stick, but I agree with you, nmvb & Alderin, it's an interesting idea.
nmvb3 years ago
You should add wings on either side and it would fly around the room
3 years ago
Paint it gold... it's a Snitch!
mikenaly3 years ago
Just a thought here, but you said you were guessing on the rotational speed of your ping pong ball in the video. To me it looked like it was holding it's ground pretty solidly. You could evenly space black (or reflective) stripes around the ball and set up a LED to light the side of the ball and a photo diode to pick up the LE's reflection. Next count the pulses coming out of the photo diode, divide by the number of stripes reflecting the LED, and you have an accurate measurement of the balls rotational speed at any given moment.
makendo (author)  mikenaly3 years ago
Nice idea. I can get an idea of the minimum rpm by inspecting the video - when the ball winds up, the markings are an indistinguishable blur from one frame to the next, suggesting that it's spinning at at least 25 rps (1500 rpm). Strobing a marked ball at different rates would allow you to do this sort of estimate more accurately, but your method would be excellent if you wanted the exact rpm.
NiVofHiR3 years ago
Perhaps using a syringe you could inject the liquid nitrogen into the ping-pong balls' cavity? ...and it might spin longer/faster??
3 years ago
I believe the warm air inside the ball contracting in the LN2 would suck more liquid into the ball than trying to force it in with a syringe. If that were tried, the LN2 would hit the relatively warm air inside and try to vent back out the hole. Could be inconvenient if the syringe needle sealed the hole.
makendo (author)  MTJimL3 years ago
It's impossible to draw the LN2 into a syringe, unfortunately, unless the syringe is at -196C itself.
3 years ago
I was wondering if it was even possible to suck the LN2 into a syringe.
just a curious q......... does the flower stay crystallized or does it go to its original stuff........ i have seen an experiment where tey put a frog in liquid nitrogen and it gets back after unfreezing (Hibernation)
3 years ago
I highly doubt that any frog could survive being frozen in liquid nitrogen.
Maybe you were thinking of the Wood Frog?
What flower?
Some frogs can survive freezing, but with LN2? I don't think so.
At 0:07 we see what looks like a toe. Are you doing this experiment barefoot?!?
3 years ago
That definitely seems to be the case.

F•••s: This guy gives none.
makendo (author)  NRen2k53 years ago
Guilty as charged. Toe should not have appeared, does nothing for aesthetics of video, but it was tricky doing the experiment and filming simultaneously. And in my defense, it was a hot day and a few splashes of LN2 on bare feet are quite refreshing.
Mechenger3 years ago
Very cool. Thank you for posting.
osh1143 years ago
Where can i get liquid nitrogen?
makendo (author)  osh1143 years ago
see step 1
biochemtronics3 years ago
This is great. Thanks for sharing. It would be cool to use a ball with a dimpled surface to do this. The dimples would add lift the same way they do on a golf ball. If it was light enough and could spin fast enough it might even take flight. Now that would be cool.
makendo (author)  biochemtronics3 years ago
Cheers. I thought the dimples on a golf ball just reduced drag (necessary because of the crappy aerodynamics of a translating sphere). The aerodynamics of a rotating sphere are excellent, so I suspect dimples would just slow down the spin... they might stabilize it, though - the ping pong ball is certainly twitchy when it's spinning. Worth a try if you have such a ball, anyway.
3 years ago
The dimples on a golf ball actually do provide lift when combined with significant backspin. Below is a link to one site that explains the physics behind dimples creating lift.

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/
makendo (author)  biochemtronics3 years ago
Now I'm tempted to do the experiment and throw the ball off a high place to see if it does something interesting... :)
Kiteman3 years ago
Oh, awesome trick!
Tomcat943 years ago
This is brilliant! You should enter this in the Education Contest!
makendo (author)  Tomcat943 years ago
Thanks. Good idea, I'll do that.
tremelo683 years ago
Would have been cooler if you spilled some on your toes... Why aren't you promoting safety? Other than that, I found it interesting and educational.
mattthegamer4633 years ago
Thanks for the great experiment and physics explanation, certainly a must have for something interesting like this.