Todd's pendulum


A series of small bars, kept together by pin joints allowing them to rotate. The friction between the pin joints is very small and therefore the pendulum can oscillate for long time. The device is protected by a plexiglas box.

Clip A

Clip B

Move the pendulum away from its stable equilibrium, let it go and observe the resulting motion (Clip A). Then repeat the experiment, trying to start from the very same initial position (Clip B). Comparing the evolution in the two cases you can observe that, even if the initial position were near, the resulting motions are similar only at first, while in time their difference is more and more evident. This effect arises from the variation in the position between the two experiments. Even the smallest difference in the position from which you let the pendulum go generates a diverse temporal motion evolution.

Such a result is common to every system with many degrees of freedom: their temporal evolution becomes less and less predictable as time passes.