Surviving a cliff jump: go second*

Children and adults often ask us the question, “If I jump from a bridge into water can I throw an object at the water before I land and reduce the impact?” We tested this theory by dropping two spheres, axially aligned but distance separated, into a quiescent pool of water. The first sphere creates a cavity through which the second sphere enters. We show that the high peak force at the first few moments of the impact can be dramatically reduced. The diameter of the first sphere was varied while the diameter of the second sphere was kept constant. Through high-speed imaging and onboard accelerometers, we were able to divide the consecutive sphere entry into different regimes based on the cavity shape at the moment of the second sphere entry, revealing that the impact forces experienced in each regime are different and smaller than the single sphere case.