Structures: Or Why Things Don't Fall Down

by J.E. Gordon

With a bow or a palintonon, when the bowstring is first released, some of the stored strain energy is communicated directly to the missile as kinetic energy. More of the available energy, however, is being used to accelerate the arms of the bow or the catapult, where it is temporarily stored as kinetic energy, much as it is in the trebuchet. In this case, though, as the discharge mechanism proceeds, the moving arms are slowed down, not by a fixed stop, but by the bowstring itself as it straightens and tautens. This further increases the tension in the string, enabling it to push yet harder on the missile and so speed it on its way. Thus, much of the kinetic energy stored in the arms is recovered.