The study of pendulum motion is a classical problem in
physics and understanding the influence of fluid drag on its
decay dates back to Stokes 1. He derived a simple expression
for drag on a sphere at low Reynolds numbers, which
was later expanded on to include the effects of added mass
and other phenomena e.g., Landau and Lifshitz 2.
At low Reynolds numbers, this drag, FD s , can be expressed
as