A computer cooled by a fan contains eight PCBs, each dissipating 10W power. The height of the PCBs is 12 cm and the length is 18 cm. The cooling air is supplied by a 25 W fan mounted at the inlet. If the temperature rise of air as it flows through the case of the computer is not to exceed 10 C, determine (a) the flow rate of the air that the fan needs to deliver and (b) the fraction of the temperature rise of air that is due to the heat generated by the fan and its motor.

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