Springy Material Boosts Projectile Performance

Adding a soft, springy material to a solid projectile leads to a threefold increase in kinetic energy when the projectile is launched upward.

Figure caption 

 Throwing is as central to some sports today as it once was for hunting and warfare. A team of scientists now shows how throwing efficiency could be improved by engineering the elasticity of the projectile [1]. Their experiments show that, compared with a rigid projectile, one that includes some soft, springy material can receive up to 3 times more kinetic energy when launched upward. The results could be used to design better projectiles in sports and could lead to shoe designs that enhance energy transfer during walking and running.

Throwing involves a complex chain of events that couple the movements of the limbs, skeleton, muscles, and tendons to give the projectile kinetic energy. These movements are not driven purely by muscle power but also involve the release of elastic energy stored in the tendons, as in a catapult.

Franck Celestini and Christophe Raufaste of the University of Côte d’Azur, France, and their colleagues have now looked at how that energy transfer might be improved in a simple model system in which a projectile is launched from an upward-moving platform. They have previously shown that timing can be crucial to efficient propulsion when the projectile is deformable and elastic. A drop of water launched upward from an oscillating platform can gain up to 2.5 times the kinetic energy of a rigid ball with identical mass when the platform vibrates at about one-third the frequency of the drop’s intrinsic oscillations [2] (see also Focus: Superpropulsion of Liquid Drops).

The team has now run similar experiments on rigid objects with an added elastic component as a way of studying throwing efficiency in a simplified system. The researchers used a spring-loaded platform to deliver upward thrust to cylindrical projectiles 12 mm wide and 15 mm tall. The rigid portion of each cylinder was made of hard plastic, and the researchers added a squishy layer of gelatin hydrogel of various thicknesses (from about 6 mm to the full height) to the bottom of some projectiles. They determined the efficiency of energy transfer by measuring the heights to which the projectiles rose.