The fastest bird is the spine-tailed swift, which reaches speeds of 171 km/h. Suppose that you wish
to shoot such a bird with a .22-caliber rifle that fires
a bullet with a speed of 366 m/s. If you fire at the
instant when the bird is 30 m directly overhead, how
many meters ahead of the bird must you aim the
rifle? Ignore gravity in this problem.

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

the correct answer is "inertia"