I know I saw this demo somewhere. Maybe it was at an AAPT conference a few years ago. I have always wanted to build this, but never got around to it. Until now. Here is the demo (it is easy, you should make one too)
So, how does this work? I think the simplest explanation is that the drinks do not spill because the string can only pull in the direction of the standing glasses. A slightly better explanation is that the string lets the tray rotate so that the sum of the acceleration and the gravitational field is in the direction of the open ending of the cup. I am still not happy with that. Let me start with a case where the water would spill. Suppose I just hold the cup in my hand and stop really quick. Here is a force diagram for the cup with water.
I don’t know why I broke the hand force into two components. Anyway, remember when I talked about fake forces? Basically, if I want to pretend my reference frame is not accelerating when in fact it is, I can add a fake acceleration where:
So, the diagram for the glass looks a little different if I use a fake force.
But, I really want to look at the water. Here is a force diagram for a little piece of water near the top of the surface.
I am not going to talk about the buoyancy force, but it is essentially the water below that ‘piece’ of water pushing it up. There is an interesting thing in common to both the fake force and the gravitational force from the Earth. Both pull on all pieces of the water. Compare this to the forces the glass exerts on the water. The glass can only push on the parts of the water that it is touching. So effectively, there is a new gravitational force (represented by the red vector). The water is level perpendicular to this effective gravity. If this new level makes the water go over the rim of the glass, it spills.
Now, what about the cool serving tray? I guess you can already figure out that the one-string tray adjusts itself so that it is perpendicular to the effective gravity. This happens because a string can only pull in the direction of the string. The result is that the water is accelerating, but in the direction the cup points.
This is essentially the same as the “swing the bucket of water over your head” demo, but more cooler (in my opinion). Ok, one more thing. You CAN spill the water if you are not careful. If you slip and let the tension in the string go slack, the stuff on the tray will fall. See this example.Go Back to Top. Skip To: Start of Article.