If you press a book flat against a vertical wall with your hand, in what direction is the friction force exerted by the wall on the book?.

force tends to be low and long diustance masses are high

pecialized keyboards

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