A police man hidden at an intersection observes a car that ignores a stop sign, crosses the intersection and continues at a constant speed. police man starts off 2.0s after the car, accelerates at 6.2m/s2 until the speed is 110Km/h ,then continues at this speed until he catches up with the car 1.4km from the intersection. How fast was the car traveling?

can be answered using momentum.

momentum = p

p1= mass1 * velocity1

p1 = p2

therefore

p1 = 6kg * 4m/s

p1 = 24kgm/s

p2=m2 * v2

24 kgm/s = 7kg * v2

v2 = 24 kgm/s / 7kg

v2= 3.42 m/s

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