A bullet of mass 6g travelling at 120 meter's per second penetrates
deep into a fixed target and is brought to rest in 0.01
seconds. Find;

U = 120 m/s

v = 0

t = 0.01 s

a = (v-u)/ t = -120 / 0.01 = -12000 m/s

a) S = (v² - u²)/2a = -120² / 2*(-12000) = 120 / 200 = 0.6 m

distance of penetration is 0.6m

b) force = ma = 0.005×12000 = 60N

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