I'm trying to explain to a coworker some fallacies in a thought problem he came up with that has severely confused him on some basic principles of clasical physics... but I am having some issues. He is capable of understanding the math, but he is not capable of seperating intuition from mathematical equations. His thought problem, as presented to me, was this (Using the frame of reference of a person on the ground): Edit ... I wanted to present the thought problem he had to see how others tore into it... instead ... I'll simply ask this question (same frame mentioned above). If stationary person (A) shoots a bullet from a gun and we record the muzzle velocity as n meters per second, what will the muzzle velocity of a similar bullet (same starting potential energy in the accelerant) shot from a similar gun (same barrel length) if fired by a person (B) on a superfast train which is already moving at a velocity equal to that of the muzzle velocity of the first bullet (n m/s) in terms of n? I'm pretty sure I've got the right answer but don't want to pollute the question with my own mathematically derived assumtions the way my coworker was polluting his version of this problem with his own (magically? derived) assumptions. Looking forward to your equations and examples.