The center of mass of a structure moves independently of any forces arising within the structure. An illustration of this independence is the oft-quoted case of the center of mass of a shell continuing along a parabolic trajectory even after the shell has exploded. 3D Mechanics can simulate this classic example.

In figure 1 we see two masses bound together by a rigid connector. Hidden from view is a second connector which also joins the two masses. However this connector is elastic and is highly compressed. Since the rigid connector has been highlighted, it can be destroyed simply by pressing the Delete button on the keyboard. Once this key has been pressed, the elasic connector will then be free to drive the two masses apart as it returns to its original length. So this structure amounts to a bomb which we may detonate at will. In the figure the bomb is moving along a parabola under the influence of gravity.

Figure 2 shows the situation a few moments after the bomb was detonated. We see the two masses moving rapidly apart and the center of mass moving just as it would if the shell were still intact. The simulation of the motion of the shell depends only on Newton's three laws and not on any statements of how the center of mass should behave.