1. The problem statement, all variables and given/known data Two blocks move along a horizontal frictionless surface and collide head-on with each other. The mass of the first block, moving to the right with speed v is m, and the mass of the second block moving towards the left with speed 3v is 2m. After the collision, the block of mass m is moving at a speed 2v. Find the speed u of the block of mass 2m after the collision. The answer shall be a numerical value. 2. Relevant equations Newton's second law and kinetic energy equation. 3. The attempt at a solution I have tried my best to use the second law, but of no avail, considering the the answer should come out to be a numerical value.