Impact force reduction by consecutive water entry of spheres

Free-falling objects impacting onto water pools experience a very high initial impact force, greatest at the moment when breaking through free surface. Many have intuitively wondered whether throwing another object in front of an important (like oneself) before surface may reduce this force. Here, we test idea experimentally by allowing two spheres to consecutively enter and measuring forces on trailing sphere. We find that acceleration reduction sphere depends dynamics cavity created first relative timing second impact. These combined effects are captured non-dimensional `Matryoshka' number, which classifies observed phenomena into four major regimes. In three these regimes, is reduced up 78\% quiescent Surprisingly, one force dramatically increased more than 400\% worst case observed. explain how various stages evolution result alterations multi-body entry problem.