A cart is traveling along a track and slows down due to friction, why did it slow down

Friction can slow things down and stop stationary things from moving. In a frictionless world, more objects would be sliding about, clothes and shoes would be difficult to keep on and it would be very difficult for people or cars to get moving or change direction.

protons in

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