The movie illustrates the difference between evolution of three point
particles and three heavy spheres, initially aligned horizontally. The
settle in a viscous fluid (Stokes flow) under identical vertical
forces. Their relative motion is shown in the frame of reference moving
the middle (red) particle.
M. L. Ekiel-Jeżewska and E. Wajnryb
Rev. E 73
, 046309 (2006)