Un cañón de electrones dispara electrones (q = -e, me = 9.1 × 10 -31 kg) hacia una placa metálica que está a 4.0 mm de distancia en el vacío. El potencial de la placa es de 5.0 V menor que el del cañón. ¿Cuán rápido se deben mover los electrones al salir del cañón si deben llegar a la placa?

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