It takes 300 newtons of force and a distance of 20 meters for a moving cart to come to a stop. how much kinetic energy did this cart have?

As we know from work energy theorem

Work done by all the forces is change in kinetic energy of the system

here we know that finally it stops so final kinetic energy of the cart here is ZERO

while we need to find the initial kinetic energy of the cart which is equal to the work done

so here we can say

also we know that work done by the force is given by

here we have

F = 300 N

d = 20 m

so kinetic energy of the cart will be 6000 J

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