The atmospheric pressure is 1 ATM. What does it mean?​

the atmospheric pressure is generally expressed in terms of the height of the mercury column.at normal temperature and pressure, the barometric height is 760mm at sea level which is taken as one atmosphere. thus atmospheric pressure is also expressed in a unit atmosphere where 1 atm= 0.76m of hg. 9

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