A 200 kg car is moving rightward at 3 m/s. What speed should a leftward moving 300 kg car have so that the two collide and come to a stop? (​Set the Blue bumper car mass to 200 kg and velocity to +3 m/s. Set the Red bumper car mass to 300 kg and the bumper type to Locking. Adjust the Red car velocity so that the two come to a stop after they collide).

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

this would be about 43%