When two objects collide their velocities change in response to the compressive (pushing) force between them.
The difference in (normal) velocities between the objects is thus eliminated or reversed. However, for non-rigid
objects collisions are more subtle. Surprisingly, when a long chain moving lengthwise collides with, say, a wall
or floor, the chain can be pulled into the wall (instead of pushed away) with the approach velocities between the
wall and chain increasing in time (rather than not changing or decreasing). Why? The incremental bits of mass
that are colliding are slowed by the wall. But they can also be slowed by the remaining chain, thus speeding the
remaining chain. The extent to which the impulse which slows the colliding bits comes from the wall or from
the remaining chain determines the acceleration of the remaining chain. We show theoretical limits on how
much a chain can be pulled into something with which it collides, some chain link designs that lead to these
limits, and experimental results which show the sucking of one of these designs into a wall.