I conducted an experiment in my youth, creating a 'sand pendulum' hanging from the bedroom ceiling to a bowl of flour. I did not set the pendulum into motion, but rather wanted to see if the force of gravity was constant or shifted over time. I don't recall if I checked within or after 24 hours. However, when I checked, I found my pendulum had traced a small circle in the flour.

I cannot find any information on whether this experiment (involving a non-swinging sand pendulum) had been conducted previously and how it may relate to the known physics of pendulums and Earth's rotation.

Please let me know, Conor H. Murray, PhD (neurobiologist)

  • 2
    $\begingroup$ Are you familiar with the Foucault pendulum? $\endgroup$ – Adrian Howard Apr 20 at 21:45

Theoretically, if there is no friction on the pivot point at the ceiling, and also no air drag around the string and bob, the pendulum at rest would seem making a cicle in the floor as the Earth rotates. The idea is that the rotation can not communicate to the pendulum in the complete absence of friction.

But I'm afraid that a very small friction is enough to destroy this effect. The pendulum rotates with the earth as any object in the bedroom and nothing special is seen.

So, if there was a circle on the floor, it had probably another reason.

When the pendulum is swinging, something different happens. The friction effect is to provide a torque to an object with an angular momentum. Instead of rotating the plane of swing, the torque adds a small transversal swing. The net effect is that the path of the bob is a kind of very excentric elipse, but keeping the same endpoints (where the speed is momentarily zero). As the Earth rotates, that endpoints shift, what can be observed clearly after 1 hour of oscillations in the Foucault pendulum in the Paris Pantheon.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.