A 6.0 kg wagon moving right at 3 meters per second on a frictionless path collides and to sticks a 4.0 kg wagon moving left at 12 meters per second. What is the final speed of the wagon?

where’s the list ? the question is incomplete

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