When we push even a rigid object we deform it to some degree. While this deformation initially is confined to the point at which the force acts, it quickly spreads through the entire structure. The velocities created as the object changes shape determine its subsequent motion. Using 3D Mechanics we can watch as velocity propagates through a structure and so better understand why it moves as it does.

The cyan vector shows that the beam in figure 1a has been given a momentary shove at its midpoint. In figure 1b we see that the middle mass has moved to the right, stretching its connections to the remainder of the structure which so far is unaffected. In figure 1c the connectors have begun to return to their original lengths, symmetrically spreading the effects of the force through the structure. In figure 1d the beam has almost regained its original shape. Because the velocities spread symmetrically from their point of origin the beam simply translates to the right.

Now we will push midway between the center of the beam and its upper end. We see that this change results in an asymmetric spread of velocity. Because there are more masses below the point where the force acts than there are above it, velocity tends to accumulate at the top of the beam and drain off towards its bottom. The result is rotation as well as translation. The additional rotation does not mean that more motion has been put into the beam than before; the rotation develops due to a shifting of the previous velocity distribution and not an increase in the total velocity imparted to the beam.

Finally we will push the beam at its top end. In this case the velocity distribution is so asymmetrical that when the beam snaps back to its original straight shape the lower end actually moves backward.