The Universe and Newton: A Love Story

A friend of mine recent asked me “Can you fire a gun in space? And if so would the bullet keep going forever at the same speed?”  The answer for this might not be as simple as one might think. First let me answer the first part of the question.

When a gun is fired a firing pin will strike the primer that will ignite the propellant (in this case gun powder). In older weapons you would have to pack the gun powder in behind the bullet. In both cases the gun power is ignited, and the bullet is dispelled from the weapon. The difference is that in a modern bullet casing, everything is sealed. Since you need oxygen for an ignition, older weapons could not fire. But modern weapons have oxygen inside of the casing allowing them to be fired in a zero oxygen environments.

Now what happens after the bullet is fired is another matter altogether. First let me point out that for every action there is an equal and opposite reaction (Newton’s Third Law of Motion). And when the weapon is fired, the recoil will be equal to the force of the propelled bullet. So assuming you aren’t being held in place by something you will fly in the opposite direction. 

Apart from the fantastic visual I get of an astronaut hurtling through space clutching a rifle, there is the matter of the bullet itself. According to Newton’s first law of motion; an object in motion will stay in motion unless acted upon by an external force. Normally the external force that slows down a bullet is friction and gravity, or a body… But in the vacuum of space there is no friction. But there is gravity.

You see, every point of mass attracts every other point of mass (Newton’s Law of Universal Gravitation). That is why the moon orbits the Earth, and the Earth orbits the sun. So all throughout space, gravity is acting upon everything including something as small as a bullet. What is confusing is that, those exact forces of gravity are still unknown! If every point of mass is attracted to every other point of mass then logically the universe should be converging, essentially collapsing in on itself. Yet the universe is expanding! These forces of nature are still a mystery to us. So the effects it would have on a bullet are unfathomable.

Considering the mass of the bullet the forces of gravity acting upon it are probably too small to notice. But that doesn’t mean that they are not there. Assuming the bullet doesn’t get too close to a planet and get caught in orbit, or hit the planet, or a rock floating in space, or even an alien! It will eventually slow to a stop and become caught in the currents of the universe.