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.
<p>I made Simple interactive ping pong game</p><p><a href="http://www.instructables.com/id/Free-Interactive-Projection-Pong-game-Water-effect/" rel="nofollow">http://www.instructables.com/id/Free-Interactive-Projection-Pong-game-Water-effect/ </a></p>
thats so cool! but... i personally think its is spinning at 9,997.643 RPM... <br>
<p>nah 9 999.999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999 RPM</p>
So sweet
great job man was it hard <br>
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!) <br>I found that it needed a breath of warm air, or a quick grab with the hand to kick it off. <br>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? <br>
If it's spinning around its horizontal axis instead of its vertical axis, it could spin fast enough to create the &quot;Magnus Effect&quot;... making it &quot;fly&quot;. Pretty cool either way!<br> <br> Magnus effect:<a href="http://www.instructables.com/id/The-50-cent-Toy-Hovercraft-Helicopter-Demonstra/" rel="nofollow">http://www.instructables.com/id/The-50-cent-Toy-Hovercraft-Helicopter-Demonstra/</a><br> <br>
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! <br>
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.
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 &amp; Alderin, it's an interesting idea.
You should add wings on either side and it would fly around the room <br>
Paint it gold... it's a Snitch!
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.
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??
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.
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)
I highly doubt that any frog could survive being frozen in liquid nitrogen. <br>Maybe you were thinking of the Wood Frog? <br>http://www.youtube.com/watch?v=UvCdOXG2rPo
What flower? <br>Some frogs can survive freezing, but with LN2? I don't think so. <br>
At 0:07 we see what looks like a toe. Are you doing this experiment barefoot?!?
That definitely seems to be the case.<br> <br> F&bull;&bull;&bull;s: This guy gives none.
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.
Very cool. Thank you for posting.
Where can i get liquid nitrogen? <br>
see step 1
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.
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.
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. <br> <br>I remember decades ago seeing a concept drawing in Poplar Science of a giant rotating dimpled sphere being used as a flying crane. <br> <br>http://www.todayifoundout.com/index.php/2010/12/why-golf-balls-have-dimples/
Now I'm tempted to do the experiment and throw the ball off a high place to see if it does something interesting... :)
Oh, awesome trick!
This is brilliant! You should enter this in the Education Contest!
Thanks. Good idea, I'll do that.
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.
Thanks for the great experiment and physics explanation, certainly a must have for something interesting like this.

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Bio: By day, I teach and document solutions to problems. By night... hmm. I should probably get out more.
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