There are several illustrative models that are easily programmed
and serve to clarify some of the problems discussed.
1. Two equal masses are attached to a spring and rest on a horizontal
frictionless surface. One mass rests against a fixed block. The spring
is compressed and the system released.
M-/\/\/\-M:B
Plot the ensuing motion of the masses and the center of mass.
(The masses are PERMANENTLY attached to the spring.)
The simplest type of integration of Newton's Second Law will work.
2. Extensions of the basic problem:
a. Two unequal masses
b. Three masses and two springs, various intitial conditions.