A small rock is thrown vertically upward with a speed of 18.0 m/s from the edge of the roof of a 40.0 m tall building. The rock doesn't hit the building on its way back down and lands in the street below. Air resistance can be neglected. Required:
a. What is the speed of the rock just before it hits the street?
b. How much time elapses from when the rock is thrown until it hits the street?

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