Using a maximum allowable shear stress of 70 mpa, find the shaft diameter needed to transmit 40 kw when (a) the shaft speed is 2500 rev/min. (b) the shaft speed is 250 rev/min.

a skateboard rolling across the street - apex

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