January 01, 1998 Filed in: Demo Corner

Bricks, books, or metre sticks are all you need for this neat demonstration. As illustrated, the top brick projects by half its length and subsequent bricks project 1/4, 1/6, 1/8, et. Brick lengths. After

There are numerous complicated ways of determining the maximum projection for each brick. Even though the physics is simple, the equations soon become unwieldy, requiring clever techniques that I have forgotten. There is, however, an easy way to look at the problem. Just consider the total projected mass at various levels in the stack. Above the second brick from the top, there is one-half brick mass projected. Above the third brick from the top, there is the half brick mass just mentioned, plus two quarters (one quarter from the second brick and another quarter from the top brick). Above the fourth brick from the top, the total projected mass is one half plus two quarters plus two sixths. This arrangement ensures that the cantilever exactly balances on the front edge of each brick because at each stage an additional half brick mass is added to the total overhang. The centre of mass of the projected portion is balanced by the centre of mass of the rest of the stack. (You might want to think in detail about how this scheme works once two or more bricks are completely projected beyond the table edge, so that they can’t contribute any more projected mass.)

It is hard to imagine how a loose stack of bricks could extend out infinitely far. It should make for a lively discussion.