All, I started this strange little thought experiment this morning, but I'm not quite sure what I'm missing. Here's the problem I solved (this is not homework, BTW): Find the speed at which a super hero (mass=76.0 kg) must fly into a train (mass = 19,537 kg) traveling at 35 m/s to stop it ... ANSWER: ~9,000 m/s. What I'm wondering is this: How does a problem such as this relate to work and time? Am I correct in saying that there is (technically) no work done because the kinetic energy of the superhero and train cancel each other completely (ignoring that lost to sound, heat, etc.), and the collision takes place in such a short time that the distance is "0"? Secondly, is it possible to calculate force and time in this problem? I'm assuming time would be extremely small because the force is so large (I'm thinking about the impulse equation here). Any help clearing up my mental state is much appreciated!