Intermediate soft layer for optimized throws*

The amount of energy transferred from a throwing device to a homogeneous elastic projectile can reach 250 % the kinetic energy of a rigid projectile for the right tuning between the timescale of the thrower acceleration and the eigenperiod of the projectile [1].
Here we show that the kinetic energy of a rigid projectile is greatly increased thanks to a geometrical change: the addition of an intermediate soft layer between the throwing device and the projectile [2]. Experiments have been performed with a centimeter long rigid polymer and a soft hydrogel whose length has been varied. The throwing device has a harmonic motion and an initial acceleration of about 10 times standard gravity.
Simulations and analytical models based on the resolution of the wave equation in the soft part show that the optimal energy transfer emerges for the right time tuning between the throwing device and the projectile as well as for the right proportion of soft material. From the measurements we show that the rigid projectile can obtain 400 % the kinetic energy it gets without a soft layer.

[1] Raufaste et al. , Phys. Rev. Lett. 2017
[2] Celestini et al. , Phys. Rev. Appl. 2020