# How fast does a bullet travel on the moon?

I was in school today and we were talking about how their is little gravity on the moon and all that fun stuff. So it got me thinking, if you shoot a gun on the moon how fast will it travel? And what if you shoot it at someone? Would it still pierce the skin and stuff?

One could certainly fire a gun on the moon. You wouldn’t have the pressure of the atmosphere resisting the bullet in the barrel. This is 14.7psi on earth at sea level. Of course the chamber pressure for modern ammunition might be anywhere from 20,000-50,000 psi so the difference would be negligible. I think muzzle velocity would be about the same as it is on earth. The difference would be that the bullet would not lose velocity during it’s flight - no drag - and vertical drop would be at an acceleration rate 1/6 of that on earth. A bullet fired on the moon could travel many times farther than on earth and terminal velocity would have the same magnigtude as when it left the barell. That is to say it would be going just as fast but in a different direction.

It might travel slightly faster out of the barrel because of the lack of air resistance, but I think that would be negligible. However, it would travel much much farther before landing, both from the vacuum and lower gravity.

On the other hand, I don’t think standard bullets have their own supply of oxygen for the propellant. That would be a problem.

The powder contains an oxidizer, it’s no problem.

Since there is no oxygen in the Moon’s atmosphere — or even any atmosphere, come to mention it — there is therefore no combustion, and the gun won’t work. So the fussy, snarky answer to your question is zero, in whatever speed units you like.

But if you bypass this obstacle somehow, the bullet would then fire with the same muzzle velocity as it would on Earth. Gravity doesn’t enter into it.

However, the bullet will fall to the ground more slowly. It will take roughly 2.5 times longer to hit the ground, and travel 2.5 times further, as it would on Earth — assuming you compare the same type of shot that is, say parallel to the ground in both cases. Note that this figure assumes away air resistance, which is a good enough assumption on the Moon, though not on Earth.

But hey, since there’s no air on the Moon, you can’t fire the gun! Damn, we’re back where we started.

That’s what my teacher figured. Or like since there is really no air it would just fall to the ground.

[anal retentive nitpick mode]Modern ammunition uses propellant. Granules of nitrocellulose, sometimes combined with nitroglycerine with a deterrent coating to regulate the combustion rate. Those compounds are self oxidizing. Propellants are not classified as explosives. It you have a pile of it and light a match to it you get a vigorous fwoosh but not a kaboom. Confining the propellant in a closed chamber allows pressure to rise in a small fraction of a second which in turn dramatically speeds up combustion.

The term gunpowder is only correctly applied to a mixture of the fuels charcoal and sulfur and the oxidizer potassium nitrate, commonly called saltpeter. Black powder is an explosive. A large enough loose pile will go kaboom if ignited.

I’m not certain if the flame dynamics in the flash pan would allow a flintlock to work reliably but I see no reason that virtually any other firearm could not be used in a vacuum.

If this is true, then it ruins the humor of my post — but so be it. I had just assumed the explosion used oxygen from the surrounding air.

Your teacher needs to spend some time on SDMB or take physics 101. A bullet or a thrown rock does not fly in the same sense an airplane does. If you tossed a rock on the moon with velocity X its path may not look that much different from one thrown on earth except it would not fall as fast. If you tossed a paper airplaine with the same velocity it would be different from one thrown on earth as it would match the “flight” path of the rock.

All modern weapons use ammunition with oxidants. They wouldn’t work otherwise, even IN air, because the firing chamber would not permit enough oxygen to rush in to supply the explosion. Guns would work fine in a vacuum.

One thing; if you’re angling your gun upwards, and it’s sufficiently powerful, your bullet might never land.

If you shot a gun on the moon, it wouldn’t move very far if you were holding it properly.

Ohhhh, you meant the bullet, sorry. Assuming you could get the gun to fire with in an airless environment, it would move faster and shoot farther than that bullet shot on the Earth, because there would be very little air resistance. Even discounting air resistance, it would also fly further because the moon’s surface gravity is significantly less than the Earth’s, so the bullet would fall to the ground slower than on Earth. Yes, it would still pierce the skin.

So then you might be able to fire a bullet that hit the earth?

1/6 gravity, but 1/4 the diameter? So… the moon is denser than the earth?

I believe that the Earth is denser than the Moon. Remember that gravity falls off as the square of the distance between the two objects.

v = volume, m = mass, g = gravity, ρ = density

v = 4/3 π r[sup]3[/sup]
m = ρ v
g ~ m/r[sup]2[/sup]

g[sub]E[/sub] = ρ[sub]E[/sub] 4/3 π r[sub]E[/sub] = 6 g[sub]M[/sub]
g[sub]M[/sub] = ρ[sub]M[/sub] 4/3 π r[sub]M[/sub] = ρ[sub]M[/sub] 1/3 π r[sub]E[/sub]

ρ[sub]E[/sub] 4/3 π r[sub]E[/sub] = ρ[sub]M[/sub] 2 π r[sub]E[/sub]

ρ[sub]E[/sub] = 3/2 ρ[sub]M[/sub]

falls off chair in confusion

Well, trying to write out the equations in English:

v is the volume of a sphere
m is the mass of a sphere
g is the accelleration of an object due to the gravity of a sphere
ρ (rho) is the density of a sphere
r is the distance from an object to the center of a sphere
π (pi) is…well…pi. 3.14159…etc.

The volume of a sphere is 4/3 π times its radius cubed. A sphere with a radius twice the size of another sphere’s will have 8 times the volume of the other.

The mass of a sphere is the density of the sphere times its volume.

The acceleration of an object due to gravity of the sphere is some arbitrary constant times the sphere’s mass divided by the sphere’s radius squared. An object that’s half as far from a given sphere than another object will feel 4 times the pull from that sphere as the other does.

Earth’s gravity at its surface is its density times its volume divided by its radius squared, and is six times as strong as the Moon’s gravity.
The Moon’s gravity at it’s surface is its density times its volume divided by its radius squared, where its radius is 1/4 the size of Earth’s radius.

I set the equation of Earth’s gravity equal to six times the equation of the Moon’s gravity, and divide out π and Earth’s radius, and then divide both sides by 4/3, to get that the Earth’s density is 1 1/2 times the density of the Moon.

This is, of course, a gross estimation, but then again, so are the starting figures – 1/6 and 1/4.

If Reality Chuck does a SF story that involves flintlocks on the moon, we will know where to place the blame!

The Earth is in fact the densest of all planets and moons. It is the densest large object in the solar system except the Sun itself.

Okay, forget everything written so far about how to calculate volume. It’s irrelevant. Here’s why the Moon has so much gravity when you’re standing on it.

The moon is actually only about 1/80th the mass of the Earth, weighing about 74 quintillion kilograms, while the pudgy Earth weighs about six sextillion kilograms. That makes the Moon in fact much less dense - like 40% less dense - which always confused me, too, since it doesn’t seem to match up with the notion that the gravity is one sixth as strong. I mean, logically, if it’s only one eightieth the mass AND less dense, the gravity should be less than 1/80th as strong, right?

It’s just that you’re so much further from all that mass on Earth. While standing on the surface of the Earth, you’re right on top of some of that mass (the part of it right under your feet) and 13,000 KM away from some of it (the part of it on the opposite side of the world.) On average you are about 6,360 kilometers from the Earth’s total mass. On the Moon you are only about 1,736 kilometres from the centre of its mass. It’s just that, in effect, you’re a LOT closer to the Moon - about 3.66 times closer.

The moon does in fact have only about 1/80th the gravity of Earth - but gravity squares with distance, right? So by being closer to the Moon’s centre of gravity, there’s more gravity than if you were as far as the Earth’s radius. You’re 3.66 times closer.

3.66 squared is 13.3956, multiplied by one eightieth is about 0.167 - ONE SIXTH. Works out perfectly. Ain’t that cool?

If the Earth were squeezed to the radius of the Moon, and you were standing on it, you would be 13.3956 times heavier - in other words, you’d be really dead.

You can do this trick with any planet. Jupiter is about 317 times heavier than the Earth. So if you were standing on the surface (well, it doesn’t really have a surface, it being a gas giant, but just pretend) you should be 317 times heavier, right?

No, not even close. Jupiter is eleven times wider. Eleven squared is 121 - 317 divided by 121 is about 2.6. You’d be 2.6 times heavier - still pretty uncomfortable, but not flattened.

To sum up:

Let’s say a gun has a muzzle velocity of 2000 ft/sec. Fired on the Earth, the bullet will leave the muzzle and begin to decelerate due to air resistance. At the same time, the bullet will begin to drop due to gravity. Eventually, it will hit the ground.

If there were no air at all, such as on the moon, the bullet would leave the barrel at the same speed (air resistance is negligible on Earth up to this point). But once out of the barrel it will continue at exactly muzzle velocity until it impacts the ground.

Now, if you shoot the bullet horizontally out of the gun, and drop another one straight down at exactly the same instant, both bullets will hit the ground at the same time - it’s just that one went some distance because it is moving horizontally while falling. Measure the time it takes for the bullet to hit the ground, multiply it by the muzzle velocity, and that will tell you how far the bullet will travel.

(okay, a minor nitpick - since the moon and earth are curved, a bullet fired horizontally will actually fall a little further, because the earth is curving away for it as it flies)

More characteristics - a bullet fired on the moon will be EXTREMELY accurate. Bullets fired in the atmosphere are jostled by wind, rising and falling air currents, and coanda effect of the spinning bullet can cause it to rise as it leaves the barrel. On the moon, the bullet will follow an extremely accurate parabolic arc from the barrel of the gun to the surface of the moon. Its flight will not vary by so much as a millimeter from that curve.

And all bullets require a cartridge with an oxidizer - if you relied on the atmosphere, you wouldn’t get an explosion - you’d get a fire. All explosives need their own oxidizer. Guns would work great in space.

I know this has already been harped on…but I’m in shock.

They let these people* teach children*! :smack: