Gunnery Chief: This, recruits, is a 20-kilo ferrous slug. Feel the weight. Every five seconds, the main gun of an Everest-class dreadnought accelerates one to 1.3 percent of light speed. It impacts with the force of a 38-kilotomb bomb. That is three times the yield of the city buster dropped on Hiroshima back on Earth. That means Sir Isaac Newton is the deadliest son-of-a-bitch in space. Now! Serviceman Burnside! What is Newton's First Law?
First Recruit: Sir! A object in motion stays in motion, sir!
Gunnery Chief: No credit for partial answers, maggot!
First Recruit: Sir! Unless acted on by an outside force, sir!
Gunnery Chief: Damn straight! I dare to assume you ignorant jackasses know that space is empty. Once you fire this husk of metal, it keeps going till it hits something. That can be a ship, or the planet behind that ship. It might go off into deep space and hit somebody else in ten thousand years. If you pull the trigger on this, you're ruining someone's day, somewhere and sometime. That is why you check your damn targets! That is why you wait for the computer to give you a damn firing solution! That is why, Serviceman Chung, we do not "eyeball it!" This is a weapon of mass destruction. You are not a cowboy shooting from the hip!
But the physics of that is entirely wrong. See Olbers' paradox. Eyeballing it is fine; if you miss you will almost certainly never hit anything - it was considered fine to eyeball AA in WWII, and the odds of hitting a friendly fighter there were worse; also there are already plenty of big rocks going fast in space.
If you want to be charitable towards it you can assume it was meant to scare those recruits into submissions so they won't waste precious ammo in deep space battles. Also, the explanation holds in orbital fights when you're shooting downwards (wrt. closest planetary body).
Gunnery Chief: This, recruits, is a 20-kilo ferrous slug. Feel the weight. Every five seconds, the main gun of an Everest-class dreadnought accelerates one to 1.3 percent of light speed. It impacts with the force of a 38-kilotomb bomb. That is three times the yield of the city buster dropped on Hiroshima back on Earth. That means Sir Isaac Newton is the deadliest son-of-a-bitch in space. Now! Serviceman Burnside! What is Newton's First Law?
First Recruit: Sir! A object in motion stays in motion, sir!
Gunnery Chief: No credit for partial answers, maggot!
First Recruit: Sir! Unless acted on by an outside force, sir!
Gunnery Chief: Damn straight! I dare to assume you ignorant jackasses know that space is empty. Once you fire this husk of metal, it keeps going till it hits something. That can be a ship, or the planet behind that ship. It might go off into deep space and hit somebody else in ten thousand years. If you pull the trigger on this, you're ruining someone's day, somewhere and sometime. That is why you check your damn targets! That is why you wait for the computer to give you a damn firing solution! That is why, Serviceman Chung, we do not "eyeball it!" This is a weapon of mass destruction. You are not a cowboy shooting from the hip!
Second Recruit: Sir, yes sir!
https://www.youtube.com/watch?v=hLpgxry542M