  The motion of weights attached to a chain or string moving on a frictionless
pulley is a classic problem of introductory physics used to understand the
relationship between force and acceleration. Here, we consider the dynamics of
the chain when one of the weights is removed and, thus, one end is pulled with
constant acceleration. This simple change has dramatic consequences for the
ensuing motion: at a finite time, the chain `lifts off' from the pulley and the
free end subsequently accelerates faster than the end that is pulled.
Eventually, the chain undergoes a dramatic reversal of curvature reminiscent of
the crack, or snap, of a whip. We combine experiments, numerical simulations,
and theoretical arguments to explain key aspects of this dynamical problem.
