When a mass collides with a spring attached to a different mass; why is the maximum compression of the spring when the velocity of both masses is the same? (Spring is massless and surface is frictionless to make things simpler) My mechanics teacher told me this when I was solving (or rather, failing to solve) something I saw in a book (for fun). Also, how would an equation of motion for the "chunk" that is the masses+plus spring come out? I felt like I was over complicating things when I did it. So basically, I'd like help understanding collisions that are inelastic...then elastic. (Can anyone help me find some similar situations I could look at?) *This wasn't for homework and I am not looking for a solution~ I'm just looking for understanding of motion. **Thanks in advanced!!!