A train travels forward at 20 m/s. A person on the train stands up and walks towards the back of the train at a velocity of 1 m/s. What is the resultant velocity of the person's motion

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

conservation of mass

mass before something happens = mass afterwards