THE MENACE OF MOMENTUM - DYNAMIC FORCES ON FLEXIBLE ORGANISMS

It has been proposed that the mechanical flexibility of many wave-swep t organisms reduces the hydrodynamic forces imposed on these plants an d animals. For example, reorientation of the organism can render it mo re streamlined, and by ''going with the flow'' a flexible organism can reduce the relative velocity between itself and the surrounding water , thereby reducing drag and Lift. Motion of the body allows the organi sm to gain momentum, however, and this momentum can apply an inertial force when the organism's motion is slowed by the deformation of the b ody's supporting structures. Through a series of mathematical models w e show that the inertial forces imposed on flexible plants and animals can be large enough to increase the overall force on the organism, mo re than offsetting the advantages of moving with the flow. A dimension less index, the jerk number, is proposed as a tool for predicting when inertial forces will be important, and the utility of this index is e xplored through an examination of the forces applied to kelps and muss els. The tendency for inertial loading to peak at certain frequencies raises the possibility that the structure of organisms can be tuned (e ither by evolution or physiological response) to avoid potentially dam aging loads.