Suppose one was on a collision course, speeding towards a tree on a bicycle. Expectedly, one would suffer a horrendous crash; the rigid tree would remain static whereas the rider would likely sustain every possible injury. Let us analyze the energy conversions. Since the tree is very massive and thus inert (as opposed to the rider), the rider is simply unable to impart any significant amount of its energy to the tree. Therefore the rider will have to absorb major parts of the energy and this explains why the tree crash is by far worse than for example crashing into a chair. The chair is less inert and much more easier to set into motion, getting its energy from the kinetic energy of the rider. What I am wondering is what kind of energy actually is absorbed when the rider crashes into the tree? It certainly is not gravitational potential energy, but other than that, what type of energy does the rider really absorb?