Inelastic collision: Momentum conserved, KE not-- How? This is a question about the text of Hailiday/Resnick/Walker 8th ed. p217 - 219. It's not a homework problem, but just about understanding the text, so I hope I am posting to the correct location, if not, please advise. In the text they say that in an inelastic collision kinetic energy is not conserved (p.217)(this is the definition of inelastic), but also that total momentum IS conserved (p.218). I can't visualize how this can be. I understand how in an inelastic collision kinetic energy (KE) is not conserved because some of the original KE goes into heat, deformation, etc. For example if you have a putty ball and it rolls in the +x direction into a mass on a frictionless surface (a one-dimensional completely inelastic collision) we can imagine that the putty ball squishes into a disk. So the KE is not conserved. What I don't understand is how, despite this deformation/heat/etc. we can still say that momentum in the +x is conserved. Since no mass is lost in the collision, isn't there a "loss of the component of v in the +x direction," in that some of the original +x v gets used to deform the ball in the y,z directions? So our new object (ball+ mass) when it keeps moving in the +x direction must have less momentum. Since this view is presumably wrong, could someone please explain how a decrease in (1/)mv^2 with m constant, requiring a decrease in v, can not lead to a decrease in mv? Thanks.