Can a pendulum at resting equilibrium position still demonstrate the Earth's rotation? 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)
 A: 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.
