Collision course

A semi-naked man lying on a bed of nails and under three concrete blocks forms the cover photo of the latest Cern Courier, the particle physics journal. The man, who is about to be hit with a sledgehammer, was photographed at a conference entitled "Physics of high energy ion collisions". The journal claims that the blow was "virtually painless" and is a good way of demonstrating the principle of conservation of momentum. By that other principle, induction, it should also work for you -- but the journal warns that "it is not advisable to repeat the experiment using a light target and only a few nails".