Can you throw an open pot of water into the air and catch it again without spilling any? The answer is YES, if you do it correctly.
The open plastic pot of water rests on a flat wooden disc, attached by three strings to a handle (picture right). The water is coloured in this picture.
In the picture on the left, I have thrown the whole assembly up in the air, in such a way that it rotates fast enough to keep the string tight. The forces of rotation keep the pot pressed against the disc and the water safely in the bottom of the pot at all times, even when it's upside down. The only tricky bit is catching it on the way down...
If you're not convinced by the still picture take a look at this Quicktime movie (about 1.5 MB).
This demonstration was judged the winner out of 16 entries in the Best Demo competition at the British Interactive Group (BIG) Event held in Newcastle upon Tyne from 21-22 July 2005. That's me on the right, performing the trick in the final of the competition.
Jonathan Sanderson has made this video of me doing the demo and explaining how it works. The video appears on the Planet Scicast website.
So how does it work?
First of all, try a little experiment. Put a tennis ball in the bottom of a bucket, pick the bucket up, and swing your arm and the bucket round and round. Even when the bucket is upside down at the top of the swing, the tennis ball won't fall out of it, as long as you're swinging it fast enough. Try it with a few centimetres of water in the bucket if you like - it will still work.
The force that presses the ball (or water) against the bottom of the bucket is the same one that you feel pressing you against the side of a car when you go round a bend. Isaac Newton told us that to make something travel in a curve we must constantly apply a force to it. It's the car applying this cornering force to you that you feel as the car goes round the bend, and it's the fact that the bucket is forcing the ball to go round in a curve that presses them together.
So to keep the pot pressed against the tray, and the water pressed against the bottom of the pot, we must force them to move in a circle. As long as we do this fast enough, all will be well. The easiest way to do this is simply to swing the tray round on the end of its strings. But another way is to give it a reasonably heavy handle, and throw it up so that the tray (with pot and water) and the handle swing around each other at opposite ends of the string. As long as it rotates fast enough to stop the string going slack, the pot will stay on the tray and the water will stay in the pot. All you have to do now is to catch it!
How to make it and do it
Making it
You need a fairly heavy handle. The reason for the handle to be heavy is not to make the demo work, but to make the handle easier to catch. As the apparatus flies through the air, the centre of mass will describe a nice smooth parabola, while the tray and the handle rotate about that centre of mass. The nearer the handle is to the centre of mass, the smoother its trajectory will be and the easier it'll be to catch.
What I built was determined largely by what happened to be lying around. Take my description as an indication of general principles, and adapt it according to your resources and ingenuity. No dimension is critical.
My handle started with a piece of aluminium tube (fished out of a river), 3cm diameter and about 20cm long. Down the middle of this is some 7mm metal tube (tent pole). The space between them is cast full of lead (church roof). You need to contrive some way to "key" the lead into the tube, because it loosens a little on cooling. The string from the tray passes down the central tube. You could try rolling up sheet lead instead; a sawn-off sash weight might also be a good option. I covered the handle with two cricket-bat grips, one over the other.
My tray is a 13cm disc of thin plywood, attached by 3 braided nylon cords to the handle. The tray is about 55 cm from the handle. Any plastic pot will do. Make yourself a soft weight to use instead of the pot during practice sessions - saves a lot of trouble!
Doing it (right-handedness assumed)
Hold the handle with the tray dangling down. Give it a gentle backswing to the right, and then swing it leftwards across your body and up into the air.
Almost certainly, you will launch it rotating into the air.
Almost certainly, the water and pot will stay on the tray.
Almost certainly, you will fail to catch it.
The key to catching the apparatus is getting it to rotate slowly enough. Don't worry about the pot falling off - even 1 revolution per second is plenty. A natural swing from the shoulder tends to spin the apparatus too fast; a lifting action just before you release it will keep the spin rate down. In the basic throw, the apparatus does one-and-a-half revolutions between release and catching. On a good day, you will release the handle to your left, swing your arm over and catch it smoothly on the right.
It seems impossible at first. Just practice, practice, practice...
Safety
Most of the hazards during performance are so obvious that I will list only one: if you miss a catch and the cord wraps round your bare wrist, you can get a nasty burn/graze. During construction, casting the lead presents obvious hazards.
How fast does the apparatus have to spin?
For the pot and water to stay on the tray, the apparatus must spin fast enough that the centripetal acceleration of the tray is greater than the acceleration due to gravity. That way, at the top of the arc, the tray is "asking" the water and pot to accelerate downwards faster than they would have done anyway under gravity.
If the centre of mass of the system is somewhere near the handle, the tray will be about 0.5 m from it (the CM). The centripetal acceleration is given by ω2r, where ω is the angular velocity and r is the radius. Our condition for the water and pot to stay on the tray is that ω2r > g, where g is the acceleration due to gravity (10 m s-2 for our purposes). With r = 0.5 m, this gives a minimum value for ω of about 4.5 rad s-1.
That is, the apparatus has to rotate at less than one revolution per second. It takes a definite effort to get it to spin this slowly.