If the Applied Force (force exerted) on both crates with the current Friction setting is 350, the acceleration a. is roughly 3.5 m/s2.
b. a constant 1 m/s2.
c. initially 1 m/s2, and then it increases as the crates' speed increases.

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

im srry but ur wrong the answer is d a smooth rock, some red dirt, and an empty bird's nest.

i believe the answer is electric potential.

explanation: