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I just saw this "trick" where a cup of water is turned over onto a table without spilling (using a piece of cardboard. After removing the cardboard from underneath the cup, the person then removes the cup in a particular way (lifts straight up and twists) and lo and behold, the water stays in it's position as if the cup were still there!? (watch the video to fully understand) Is this really possible? If so, and the real question I'm looking to have answered is, how?

After further research it appears that deionized water is needed as well.

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4 Answers 4

up vote 10 down vote accepted

Let's split up the forces, it gives an easier view on what should happen.

Horizontal Forces

What we see is a centrifugal force.

Centrifugal force (from Latin centrum, meaning "center", and fugere, meaning "to flee") represents the effects of inertia that arise in connection with rotation which are experienced as an outward force away from the center of rotation

So, outward force away from the center of rotation. This means that the water would only be able to appear in place if there is a force in the opposite direction, which is known as friction. Where is the friction if you take the glass away? There is no friction anymore, thus that video shows special effects. ;-)


Vertical Forces

There's also the gravitational force. What would keep the water up so long? Nothing.


Furthermore, if you look at his other videos, you can confirm it's CGI... :-)

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2  
So in otherwords... I've been duped. –  KronoS Feb 24 '11 at 21:44
    
The guy's good tho with the CG. Probably had a second tinted glass cup that was pulled up. –  Genie Feb 25 '11 at 3:31
    
I'm not so satisfied with this explanation; is friction the only thing that can keep water from moving outwards? Also, your explanation of gravitational forces doesn't make too much sense; I can pile a bunch of blocks on top of each other and claim that I am defying gravity by the same token. Perhaps there are forces at play that you don't know about? Maybe you could address all of the possible kinematic, intermolecular, and quantum forces that are at play and show why they aren't enough to counter the center-fleeing and downward-seeking forces you listed. –  Justin L. Feb 25 '11 at 5:46
    
In my oppinion there is almost 0 probability that it is CGI (Computer Graphics Image). The software to make a model of a static glass of water with proper caustics is commom and realistic. But to simulate the rotating water, AFAIK, there is none available to the public. It has to be a renderer and a fluid physics simulation. The option of composing frame by frame means a lot of work to keep the continuity of the rotational motion. –  Helder Velez Feb 25 '11 at 9:35
    
Feel free to correct my answer, I'm not a Physics Expert but those are just my thought of why it would be impossible. –  Tom Wijsman Feb 26 '11 at 21:12

Its a trick. There are 2 vessels:

the outer that we saw, and an inner one, transparent and soft. Stopt it at 1:48 you will see the water move out from the bottom. Under atmospheric pression the vessel merge down. The image after this moment is truncated and we can not see the remnants in the table. The water was previously set in rotational motion (before the image restarts after those words).

added:

It is not needed to explore the inner characteristics of the inner vessel but I will try. My first tough : It can be a soft plastic bag. My second try: It can be made of ice. But I suspect it is not so. Rinse the interoir of the vessel, then fridge until ice coated. Rinse again, and repeat,...or two equal vessels one inside the other separated by water and fridge. Now, with the tiny ice coating prepared. Rinse the exterior of the vessel with warm water. Wait a little and the exterior will be decoupled from the interior vessel. Start the show

added again, my final conclusion

It is a soft bag. The rotational effect is really need to keep the vertical wall under more tension and will prevent the object from immediate collapse. The centrifugal force will add strength to the vertical wall and the vertical component of force will be lower. If it was the icy vessel the rotational effect was not needed.

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Have been looking for that too, but I can't find it. 2:33 is the end of the video so there's nothing to see there, furthermore, there's no indication of any scene cuts near the moment he hovers his hand above it. The image also doesn't reveal him pulling it away and I don't see the borders. –  Tom Wijsman Feb 24 '11 at 22:05
    
Oh, it might be a border but I don't see the others. But how would he get the water to spin that fast? –  Tom Wijsman Feb 24 '11 at 22:07
    
I corrected the time, and put adicional info in the answer. I used to do stop/restart/stop/restart... –  Helder Velez Feb 24 '11 at 22:08
    
Yeah, that's where I've been looking. It's making me doubt if it's CGI or not... –  Tom Wijsman Feb 24 '11 at 22:10
    
The is no need of computer manipulation. The frames are conveniently cuted. Very clever the trick. But it is difficult in a live show. –  Helder Velez Feb 24 '11 at 22:13

We used to do the trick with the glass of water and a piece of paper when I was a child. You fill the glass, to the rim it is more successful, cover with a dry piece of paper or a bit of flat aluminum foil, put your hand flat over it, and upend it slowly. Slowly remove the hand. The water does not empty. The paper stays. The weight of the water is balanced by the ambient pressure and the suction that occurs at the top, as gravity pulls the water down and a vacuum is formed at the top of the glass.

At first glance I thought that conservation of the angular momentum given by the twist, same with a spinning top, would keep the water there ( it looks as if it is spinning in the video). But the suction from the removal of the glass (goes plop, I am an experimentalist after all) could not produce such an unperturbed shape, imho. So I vote that it is a trick, although the way the water collapses without any debris of bags is realistic, so there must be video manipulation too.

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No, It was CG. The video-maker himself said it. Here: http://forums.cgsociety.org/archive/index.php/t-957350.html

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