1. The problem statement, all variables and given/known data Two forces of equal magnitude are applied simultaneously on the ends of a bar of lenght L. In another reference frame, moving with velocity v relative to the bar (with v being parallel to the lenght L), the forces are not applied at the same time, generating momentum. I have to analyze what happens to the energy and momentum of the bar on both reference frames in two cases: 1. Considering the bar is rigid. 2. The bar is not rigid. 2. Relevant equations Special Relativity formulas F=kx 3. The attempt at a solution I was able to do it for the rigid bar, but not the other case. This is what I did: I considered F = k . dl (dl being the deformation of the bar). The bar gained an energy equal to kdl²/2, or ΔE = Fdl/2. I don't know what to do now. Also, I'm sorry for my crappy english. I tried to explain it the best I could.