Two blocks with masses 5m and 6m are on a collision course with the same initial speeds vi. The block with mass 5m is traveling to the left, and the 6m block is traveling to the right. They undergo a head-on elastic collision and each rebounds, retracing its original path. Find the final speeds of the particles. (Enter your answers in terms of vi, the initial speed of each mass.)

it's when something is going so fast it makes a loud noise, breaking the sound barrier, for example: a jet makes a loud noise, not only cause of the jet but because they reached a point where they went so fast that you can year the noise from various parts, kinda like a police car, lets say the car was going   so fast that you hear the siren of the cop car right next to you, but the car is probably a mile a way, it's cos the siren was next to you a few seconds if you understand me