1. The problem statement, all variables and given/known data You have a 10kg ball that is going 10m/s. It is about to collide with a much less massive ball that is stationary. What is the range of final velocities of the small mass? All Collisions Are Elastic Given Case 1: The large mass stops after the collision. Given Case 2: The large mass keeps moving 2. Relevant equations P=mv Pi=Pf KEi=KEf 3. The attempt at a solution So formulaically, I understand why if the large mass stops after the collision the small mass could reach an infinitely high final velocity depending on how small it was. Or it could reach a velocity of Vf=Vi=10m/s if it were just a tiny bit less massive than the original mass (say 9.9999999kg). What I dont understand is for the other case where the mass keeps moving. If the large mass keeps moving, the small mass has to move at at least the speed of the large mass. So if the smaller mass was just a tiny bit smaller than the large mass, then its final velocity could be Vi/2 = just about 5m/s (right?). But if the smaller mass were infinitely small, I dont get why it's maximum velocity would be 2v. I Can run it through the formulas since momentum and kinetic energy is conserved, but intuitively i dont understand. So A) Yes the smaler mass but be going at least as fast as the larger mass otherwise the larger mass would be "going through" the smaller mass But how, if the smaller mass is infinitely small does it make sense that it could not go faster than 20 m/s or at 2vi? What relationship am i missing here?