10,000 Rpm Ping-pong Ball




Introduction: 10,000 Rpm Ping-pong Ball

About: Analog maker dabbling in digital manufacture

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 a guess, but we are definitely talking some serious rotational velocity. It speeds up as the liquid nitrogen inside is consumed.

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.

Update 2016: The Backyard Scientist saw a gif of the video on Reddit and reproduced the experiment (he used liquid propane rather than liquid nitrogen) and estimated the rotation rate at 30,000 rpm.

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    39 Discussions

    nah 9 999.999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999 RPM


    5 years ago

    So sweet

    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?

    4 replies

    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).

    You have aroused my curiousity: What happened when you smashed the onions to cause your students to become suspicious of your actions?

    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.

    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!

    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.

    1 reply

    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.

    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.

    1 reply

    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.

    Perhaps using a syringe you could inject the liquid nitrogen into the ping-pong balls' cavity? ...and it might spin longer/faster??

    3 replies

    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.

    It's impossible to draw the LN2 into a syringe, unfortunately, unless the syringe is at -196C itself.