Light travels in a straight line at a constant speed of 3.0 x 10 8 m/s for 4.1 years to reach the earth from the nearest star 3.9 x 10 13 km away. What is
its acceleration?

Explanation:

As the velocity of light is constant so the acceleration of the light is equal to zero.

a=

dt

dv

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