Sanity Check on my Physics

Tburger said:

Quick questions for any physicists out there:

1. A bullet fired from a gun on earth, as soon as it leaves the gun barrel, is acted on by two forces: gravity and air-resistance, correct?

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Yes.

Tburger said:

2. On a planet with significantly greater gravity than earth, the effects of gravity will be greater, causing the bullet to fall faster, correct?

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Yes.

Tburger said:

3. Assuming my fictional planet has an atmosphere identical in chemical composition to earth's, what would the effects - if any - of heightened gravity be on resistance?

Thanks!

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By "identical in chemical composition" I presume you also mean the air has the same density in that gravitational field as the air does on Earth (which of course varies between sea level and mountains, and I'm sure that has its effect). In that case, there's no difference, the bullet would slow down its horizontal motion at the same rate, and of course vertically it will fall as fast as if it were dropped.

But an atmosphere of the same "chemical composition" may have a different density, depending on both gravity and the "amount" of air in the atmosphere. Now that I think about it, a planet with higher gravity is likely to have a much thicker atmosphere, as the higher gravity keeps most of the air closer to the planet and less of it will escape. Compare the atmospheres of planets/bodies with less gravity (Mars, the Moon, other moons) with those of "moderate" gravity (Earth, and Venus has a thick atmosphere) and of larger planets (Jupiter).

But the type of bullet and gun may make a difference. I barely know enough about guns to be dangerous but a rifle have 'rifling' inside the barrel, grooves that make the bullet rotate as it leaves the barrel so it spins like a top and keeps the same orientation throughout its travel. Pistols may not have this that I know of (I hear how pistols are notoriously inaccurate for shooting long distance), so for long range, the bullet might start tumbling end-over-end. In that case, it's like a baseall curve ball (or whatever other pitch make it move around), and the bullet could move up as well as to the left or right or down.

Basically you have to remember two (the only two) forces that act on a bullet. Gravity and friction. Gravity makes the bullet go in a ballistic curve. That's pretty simple, you can see how that works in pretty much any 2D archery game. Bullets are so fast anyway that normally (that means on earth) gravity does not become relevant. If gravity is noticably higher that's very simple for a shooter to compensate: just aim higher, gravity always pulls straight down.
Friction is more difficult. The denser the atmosphere the higher friction gets. And a planet with a higher gravity will also have a denser atmosphere if it has the same composition as earth's. For an atmosphere less dense you need lighter gases, such as helium and hydrogen.
Anyway, the biggest effect of friction is that the bullet slows down. The higher the friction the faster the bullet slows down. This is not directly noticable, the biggest effect will be that the bullet deviates from a straight path stronger, i.e. the guns will be less accurate. This is not as easy to compensate for as gravity since you don't know where it will drift to.
Also, this is not noticeable at all for handguns, since they are much more accurate than the humans holding them. I.e. the inaccuracies of a pistol are not because the bullet deviates, but because you cannot aim as well as with a rifle with stock, scope and tripod.
Also, if the air is much denser you might need to have thicker barrels, since the air that gets pushed out when you fire. For example flimsy barrels can burst when you fire the gun in water. This is not to say that you'd have to use noticeably thick barrels, some standard guns are built strong enough to fire under water, but if you have a weak gun that is, for example, built to be used in a vacuum (and built cheaply) might just burst when fired on a planet with thick atmosphere.

I agree with everything said and have nothing to add except another person saying "Yep, sounds right."

Pistols also have rifling. they are less accurate for a number of reasons. the one stated above being that a rifle is easier to aim is correct. also the shorter barrel make any inaccuracies in aiming that much greater (the longer the barrel the more accurate the sighting) another is that the weight of the bullet in a pistol compared to the amount of propellant (powder) is greater, meaning it will slow down faster than a rifle. the effects on a gun shot in a higher gravity environment will be the same as on earth just greater. the aerodynamic design of the bullet will minimize but not eliminate the the effects of air resistance on the bullet.

DMG

Just don't shoot guns on planets that don't have slippery air. Problem solved.

A planet with an atmosphere chemically identical to Earth's but with higher gravity would be denser at sea level than Earth's atmosphere. Air resistance would, therefore, be greater.

one thing i noticed was not in here, and should be is that gun makers actually make the barrel of the gun slant up, to correct the effects of gravity.

a gun with a perfectly level barrel will not have the same effective range of a gun with one that is slanted up.
This and the rotation of the earth is why on mythbusters, when they were firing the guns strait up the bullets were coming down all funny. like moving southwest of their rig every time.

Another quick point, a gun made for another world would slant up to correct its gravity. it would be insanely high or low (depending) on earth.

Nivarion said:

Another quick point, a gun made for another world would slant up to correct its gravity. it would be insanely high or low (depending) on earth.

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Well, for the more expensive guns you can adjust the sights, which will take care of this.

Guns aren't made slightly pointed up. They're sighted to compensate for that. The reason the bullets drifted on mythbusters was because wind higher up in the atmosphere pushes on the rounds.

If you've ever fired a .22 in windy weather, you can actually see the way the wind pushes the bullet because of its light weight(only really impacts at ranges over a hundred yards or so). But all bullets are effected the same way. A sniper has to compensate for wind, because a 10 mph wind might cause a round drift a good 4 or 5 feet over the course of a few hundred yards, just as an artillery gun would have to do the same thing.

Having been in the artillery, I can state with certainty that the guns do no adjusting. The gunners do that.

Nivarion said:

one thing i noticed was not in here, and should be is that gun makers actually make the barrel of the gun slant up, to correct the effects of gravity.

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Haha! They may do that in Texas, but in my state they slant the barrel toward the target.

barrels are so slanted upwards, its not a massive slant, but they are. the sites are more to correct you, than the gun. I.E. i always aim high, so i set my sights low.

if you want to test it, take a revolver, and fire it on its side. your shot will land to the top (of your gun) of where you shot. hold it like the gangsters do in the movies.


don't do it with a semi-automatic (or automatic), because they will jam.

This rifle folds like a hinge so you can shoot around corners. Which makes more sense than this Russian version.

I've seen the American model, but that Russian one is a whole new thing. i guess it could work, I've seen people shoot through pipes but, not sure with the rifling.

Nivarion said:

barrels are so slanted upwards, its not a massive slant, but they are. the sites are more to correct you, than the gun. I.E. i always aim high, so i set my sights low.

if you want to test it, take a revolver, and fire it on its side. your shot will land to the top (of your gun) of where you shot. hold it like the gangsters do in the movies.


don't do it with a semi-automatic (or automatic), because they will jam.

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I concur with Dommo. What would a barrel slanted slightly upwards even mean??? Slanted upwards with respect to the barrel itself, meaning the bore doesn't go straight down the barrel but is slanted? (Talk about a manufacturing nightmare. Quality control would be almost impossible to maintain.) Or slanted up with respect to the breach? Or the stock. In a word, this is patently false, and a little silly.

As any shooter knows, you calibrate your sights at the firing range to the expected distance you will be shooting, then practice compensating by moving the target known distances closer and farther. This means that the farther away your target is, the more you will have to correct by aiming above it since the bullet with fly in a quasi-parabolic trajectory. Sights and scopes have a feature that allows you to do this - it is called 'elevation.'

Handguns, especially revolvers wouldn't be adjusted for this anyway. They are short range weapons that are almost completely ineffective beyond about fifty feet. The snubnosed types are really only good inside a room. All they are designed to do is hit a man-sized target and compensating for gravity by 'slanting' would be an unnecessary expense for the manufacturer. Not to mention that bullet drop inside a room wouldn't be more than a fraction of an inch. Most handguns don't even have elevation adjustable sights. None of mine do.

As for autos, if it can't fire on it's side it is a piece of crap - and crap has been known to go to manufacture and even been issued to soldiers. Gun manufacturers test for this and good guns (aka, expensive), (Sig, H&K, Steyr, Browning, Galil, Uzi, etc,) can fire reliably in any orientation as I have proven with my own examples.

MelancholyMan said:

As for autos, if it can't fire on it's side it is a piece of crap - and crap has been known to go to manufacture and even been issued to soldiers. Gun manufacturers test for this and good guns (aka, expensive), (Sig, H&K, Steyr, Browning, Galil, Uzi, etc,) can fire reliably in any orientation as I have proven with my own examples.

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A good gun on the other hand is something like this.

MelancholyMan said:

I concur with Dommo. What would a barrel slanted slightly upwards even mean??? Slanted upwards with respect to the barrel itself, meaning the bore doesn't go straight down the barrel but is slanted? (Talk about a manufacturing nightmare. Quality control would be almost impossible to maintain.) Or slanted up with respect to the breach? Or the stock. In a word, this is patently false, and a little silly.

As any shooter knows, you calibrate your sights at the firing range to the expected distance you will be shooting, then practice compensating by moving the target known distances closer and farther. This means that the farther away your target is, the more you will have to correct by aiming above it since the bullet with fly in a quasi-parabolic trajectory. Sights and scopes have a feature that allows you to do this - it is called 'elevation.'

Handguns, especially revolvers wouldn't be adjusted for this anyway. They are short range weapons that are almost completely ineffective beyond about fifty feet. The snubnosed types are really only good inside a room. All they are designed to do is hit a man-sized target and compensating for gravity by 'slanting' would be an unnecessary expense for the manufacturer. Not to mention that bullet drop inside a room wouldn't be more than a fraction of an inch. Most handguns don't even have elevation adjustable sights. None of mine do.

As for autos, if it can't fire on it's side it is a piece of crap - and crap has been known to go to manufacture and even been issued to soldiers. Gun manufacturers test for this and good guns (aka, expensive), (Sig, H&K, Steyr, Browning, Galil, Uzi, etc,) can fire reliably in any orientation as I have proven with my own examples.

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as i was told by a gun friend of mine who has worked at a gun manufacturing plant, they bore the barrel at a two degree angle down the metal. i figured that since he worked at the factory he would have to know about it, and i have seen the on the side effect. with rifles, i see it in the way the barrel (which is bored strait) is mounted on the gun.

although i have often considered the hell it could play on your aim, as it would cause your shot to rise after a distance.

hmm, i don't remember that friend too well, he may have been an idiot. i haven't seen him in years, and its hard to make judgments from memory.


and bleh to the snub noses, they aren't good for anything. but with the others i have put led in moving rabbits outside of fifty yards so *shrugs*

ah, ignore me, iz crazziez (and good with guns)

All modern rifles and handguns designed to shoot bullets have rifled barrels--that is lands and grooves to impart spin to stablize the bullet in flight. Normal chambers and barrels are manufactured in-line and as straight as possible, with no built-in "slant."

However, sighting systems on guns may be adjusted to compensate for gravity and loss of velocity due to other laws of physics. The difference between line-of-sight and the angle of the straight barrel is probably what was meant by "slant."

Shotguns designed to shoot many pellets are usually smoothbores, and those with single barrels are also manufactured as straight as possible. Lead slugs (bullets for use in shotguns) often have grooves on their surfaces to impart spin for better accuracy.

As has been said, a world with a heavier gravitational field will slow bullets and cause a more pronounced arc in any trajectory. However, sights may be adjusted to compensate and control bullet strike as desired on Neptune or Uranus, as well as here on Terra (the earth).

Nivarion said:

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The point is that having the barrel "slanted" doesn't make the least bit of sense. You can get the same effect any number of ways which are easier. Not to mention that "at an angle" is not even a meaningful statement until it is mentioned in reference to what the angle is measured.

Nivarion said:

as i was told by a gun friend of mine who has worked at a gun manufacturing plant, they bore the barrel at a two degree angle down the metal...he may have been an idiot.

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Two degrees over two feet would be more than .8 inches. So yeah, he must have been an idiot.

Not to mention what 2 degrees would be over 100 yards...

Julie Worth said:

Haha! They may do that in Texas, but in my state they slant the barrel toward the target.

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In Texas, only if the shooter is a Vice-President.

caw