This is not a new demonstration but rather an extension of a previous post “Surface tension – floating Ping-Pong balls” The equipment is just a ping-pong ball, a cup and some water.
In the previous demonstration the ball was placed in the middle of the cup and was dragged to the side by a surface tension imbalance. Soap was then added as a surfactant to reduce the imbalance and allow the ball to float near the middle of the cup. In this extension (see, for example, various Martin Gardner books 1,2) the first part of the demonstration is the same as in “Surface tension – floating Ping-Pong balls“. However, in the second part, the ball is held in the middle by changing the curvature of the water surface rather than reducing the surface tension. The water surface curvature is changed by overfilling the cup so that the water surface curves up above the lip of the cup. In this case the minimum area occurs when the ball is centered in the cup. Again, see John Bush’s lecture notes here for a more formal discussion of surface tension.
An index of all the demonstrations posted on this blog can be found here. Don’t forget to follow @nbkaye on twitter for updates to this blog. If you have a demonstration that you use in class that you would like to share on this blog please email me (email@example.com). I also welcome comments (through the comments section or via email) on improving the demonstrations.