A 30 ky child running at 7 m/s jumps onto a 10 kg sled which was initially at rest. What will be the
velocity of the child+sled immediately after the child jumps on the sled?

hey , the answer is c

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