M. L.  Ekiel-Jeżewska and E. Wajnryb
Phys. Rev. E 73, 046309 (2006)

The movie illustrates the difference between evolution of three point particles and three heavy spheres, initially aligned horizontally. The particles settle in a viscous fluid (Stokes flow) under identical vertical gravitational forces. Their relative motion is shown in the frame of reference moving with the middle (red) particle.