This problem comes from the inelastic collision section. 1. The problem statement, all variables and given/known data "In Fig. 9-63, block 1 (mass 2.0 kg) is moving rightward at 10 m/s and block 2 (mass 5.0 kg) is moving rightward at 3.0 m/s. The surface is frictionless, and a spring with a spring constant of 1120 N/m is fixed to block 2. When the blocks collide, the compression of the spring is maximum at the instant the blocks have the same velocity. Find the maximum compression." "Fig. 9-63" is just two blocks moving to the right. The block on the right side ("block 2") has a spring attatched to the left (or "back") of it. 3. The attempt at a solution I realize you can solve it by solving for the kinetic energy lost when they're moving at the same speed (the problem kind of ruined the fun with the hint) but my question isn't about. My question is, why is safe to assume that no energy is lost by other means? (via thermal energy, sound waves, deformation, or whatever else) The problem never said anything about the collision being "ultimately elastic" (and it is in the section "inelastic collisions") Wouldn't you need to know that the collision is "ultimately elastic" (meaning kinetic energy is conserved in the end)? Are they just assuming energy loss is negligable? (That would kind of defeat the purpose of the collision being "inelastic") Or is there something inherent about colliding into a spring that implies that the collision is (at least approximately) "ultimately elsastic" (that somewhat makes sense, since the spring would "absorb some of the impact" so-to-speak, but I don't understand why they would expect you to make that assumption) Am I missing something here?