Let's draw a diagram to see what's going on:
{
Define coordinates for this problem. Put the first mass 
at the
origin 
, put mass 
at the coordinate 
.
Lets apply definition eqn. 1.17. Note that the y and z coordinates
of the center of mass are zero, the only non-trivial one is the x
coordinate. So in the x-direction
Note that if we made much heavier than than the center mass
would be shifted to the origin. If the converse where true, the center
of mass would be shifted towards . When they're equal, the center
of mass lies right in the middle between the objects.
