Balancing Cups

These stainless steel drinking bowls are called 'Balance' by the German designer, Katja Hoeltermann, and are still sold by the German company Carl Mertens.

When they are empty, the bowls lean to one side, at an angle of about 23 degrees. However when you fill the bowl with water or wine, the cup or bowl becomes upright.

How does it work?

It is clear when you examine the cup that the hollow inside is not placed centrally. So the cup has a much thicker rim on one side than on the other. This causes the empty cup to lean to one side. It can also be calculated that the cup is not solid - its density is much too low. When water or another liquid is poured into the cup, this removes the imbalance, and the cup moves to a vertical position.

If you are interested in the mathematics of how this works, a friend of ours, Dr Christian Ucke, has written a paper, explaining the mechanism. See www.ucke.de/christian/physik/ftp/lectures/balance.pdf

Presenter - Tim Rowett