Cart a, with a mass of 0.4 kg, travels on a horizontal air track at 6 m/s and hits cart b, which has a mass of 0.8 kg and is initially at rest. after the collision, cart a has a velocity of 2 m/s in the opposite direction. what is the final kinetic energy of cart b

6.4 J

Explanation:

For cart A

mA = 0.4 kg, uA = 6 m/s, vA = - 2 m/s

For cart B

mB = 0.8 kg, uB = 0, vB = ?

use the law of conservation of momentum

momentum of system before coliision = momentum of system after collision

mA uA + mB uB = mA vA + mB vB

0.4 x 6 + 0 = 0.4 x (- 2) + 0.8 x vB

vB = 4 m/s

Kinetic energy of Cart B after collision = 1/2 mB vB^2

                                                                = 1/2 x 0.8 x 4 x 4 = 6.4 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

because if the tool that you are using is not balanced it there would be false mass.