Question

what happen if we fire a bullet on moon?

Answer 1

Newton's third law dictates that the force exerted on the bullet will impart an equal and opposite force on the gun, and, because you're holding the gun, you. With very few intergalactic atoms against which to brace yourself, you'll start moving backward (not that you’d have any way of knowing). If the bullet leaves the gun barrel at 1,000 meters per second, you — because you're much more massive than it is — will head the other way at only a few centimeters per second.

Once shot, the bullet will keep going, quite literally, forever. "The bullet will never stop, because the universe is expanding faster than the bullet can catch up with any serious amount of mass" to slow it down, said Matija Cuk, an astronomer with joint appointments at Harvard University and the SETI Institute. (If the universe weren't expanding, then the one or two atoms per cubic centimeter encountered by the bullet in the near-vacuum of space would bring it to a standstill after 10 million light-years.)

Getting down to details, the universe expands at a rate of 73 kilometers per second per megaparsec (about 3 million light-years, or the average distance between galaxies). By Cuk's calculations, this means matter that is 40,000 to 50,000 light-years away from the bullet would move away from it at about the same speed at which it is travelling, and would thus be forever out of reach. In the entire future of the universe, the bullet will catch up only to atoms that are less than 40,000 or so light-years from the chamber of your gun.

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