Gravity/Physics Question

fedorable1 said:

(I posted this in the Research Section, too, so Moderators feel free to delete one if you think it's necessary).

I just have a question.

Newton's First (I think) Law states that every action has an equal and opposite reaction. So, if someone shoots a gun in a ship with no gravity, would they be shot back at the speed of the bullet?

(I say a ship, because I don't think a gun can ignite in space. Maybe I'm wrong.)

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Gravity has noting to do with it. It’s conservation of momentum. M1V1=M2V2. The momentum of the bullet (M1V1) equals the momentum of the shooter (M2V2), so the shooter’s velocity will be much less than the bullet. If the bullet has a mass of 50 grams and travels at 1000 m/s, and the shooter has a mass of 100 kg, then the velocity of the shooter is (1000 m/s) / (50/(100*1000)) = 1/2 m/s. With gravity, you wouldn’t move at all, because of the friction of your feet on the floor, but if you were wearing skates, you’d move as fast as if you were floating around in zero g.

And yes, you can fire a gun in a vacuum.

Item #1
Firing a gun can be done anywhere. All you need do is operate the firing mechanism. In a pistol, rifle or revolver, it's finger-operated lever system--a trigger; in larger guns, like a cannon, it's different but the principle is the same. Whether the projectile emerges from the muzzle of the weapon depends on where the propellent is located.

I assume you mean the pistol or rifle type of 'gun.' In such weapons, the projectile (bullet) is attached to the open end of a metal casing (shell) that contains the propellant (gun powder). Operating the trigger causes a firing pin to strike the cartridge. The percussive force initiates a chemical reaction (explosion) in the propellant. Since that chemical reaction is independent of the environment outside the cartridge, the expanding gasses drive the projectile from the casing and out of the muzzle of the weapon.

Item #2
Firing a gun inside a ship in space is probably not a very good idea because of the danger of puncturing the hull. I worry every time I see projectile weapons, especially machine guns, fired in space ships as in Stargate-SG1 or Battlestar Galactica. But of course, all is well, since it's just TV.

But let's say we have a bullet proof space ship and let's assume it's far enough from any planet so that there is negligable gravity. There is someone aboard who likes to make loud noises, has a pistol, aims it at the opposite wall and presses the trigger.

Boom! Faster than he likes, the bullet strikes the opposite wall. It bounces off and caroms about the cabin until enough of the kinetic energy imparted by the explosion is absorbed by the ship and the bullet falls to the floor, hopefully without first passing through our maniac. Although, come to think of it, if the bullet did pass through our maniac, he'd be out of the picture and we could get on with the story.

What happens to the environment? To an observer outside the spaceship, nothing happens; the ship just continues doing what it was doing before the gun was fired. That is unless the bullet-proofing fails. Then we need to discuss the principles of rocket motors. Maybe another time.

But what happens inside the ship to the maniac at the instant the gun is fired? See Julie's response above. She's right. If our maniac is floating in the air of the cabin, firing the gun will indeed push him backward. But he won't go back very fast, since there is something called inertia: An object at rest will tend to stay there until a force is applied to it. Our floating maniac is surrounded by air, and the force of firing the gun must not only move the maniac but also the air. He'll move, but not anywhere as fast as the bullet does. If he was firing a machine gun though, he'd not only be rapidly in the company of many flying bits of metal, he would soon find himself pressed against the bulkhead behind him. If there is no air inside the spaceship and the maniac has amazing breath control (or is wearing a space suit), he just moves a little faster. Kind of like putting super grease in Julie's roller skate wheels.

To be safe, give him a ray blaster.
And teach him that aiming and firing such a weapon with single bursts of red light is silly; just press the trigger and wave the death ray all about in the general direction of your opponent.
And don't fire the ray blaster at a mirror.
And wear your space suit.
Or take a different space liner to the Restaurant at the End of the Universe...

Another effect: If the shooter is floating around in zero g, firing the gun will tumble him backwards, head over heels. So he’ll have a devil of time getting off another round accurately until he hits a wall and stops tumbling. If he’s wearing a suit, it might have reaction jets to automatically eliminate the tumbling, but those would take time to act. As for the gun itself, it wouldn’t necessarily be the normal earth-type weapon, where all the momentum is imparted at once. A space gun might use bullets that are essentially tiny rockets, in which case the kick-back on the shooter would be minimal. The bullet would bury itself into the thick plastic window, with the hot jet of gasses heating it cherry red, until...blamo! The decompression sucks shooter out the window, while the quick-witted hero slams his faceplate closed and survives.



As for air pressure having any measurable effect on kickback, nope.

Sorry for "hijacking" this thread, but it's still the questions are still gun related!

Can you fire any gun underwater? I'm wondering because in some games, they allow you to fire pistols underwater, but bigger rifles won't fire at all (thinking of Half Life 2 here in case you're wondering).

How far do you think the bullet would go if you could fire it? I can't imagine it'd be very far though, since the water would probably slow it down way too much.

A sort of related question. The D-Day scene in Saving Private Ryan. It shows some scenes of soldiers stuck in the water (man, I sure would've hated to have been there), and while they're trying to lose some weight so they can get up for air, some bullets whiz by from the German guns.

The question for that is, was that scene pretty realistic (with the bullets going into the water)? How deep would you need to be for the bullet to slow down enough so it doesn't penetrate and go through you (assuming you're in the path of the bullet. I know, dodging it entirely is even better, but what if it's in your path)?

After all, bullets have the right of way according to a Private Murphy's Law hehe.

Ivonia said:

Can you fire any gun underwater?

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No. A gun that has an open powder chamber, such as a 18th Century musket won't work. Nor will a gun fire if its trigger mechanism has corroded to the point where it won't move. But otherwise, any projectile weapon that uses metal cartridges will work under water.

How far do you think the bullet would go if you could fire it? I can't imagine it'd be very far though, since the water would probably slow it down way too much.

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The density of water is orders of magnitude greater than that of air. A bullet would leave the muzzle of the gun but would rapidly slow down, imparting its momentum to the water. I suppose you might inflict damage to someone in very close range, but not anywhere the distance you get in air. For underwater use, a spear gun works well. Or torpedos.

The scene in Saving Private Ryan is fairly realistic. The density of air (0 °C, 1 atm pressure) is 1.3 kilograms per cubic meter. The density of water (4 °C) is 1000 kilograms per cubic meter--or about 770 times as great. You don't need much water to slow a bullet to a less-than-deadly velocity. (I would provide the formula to find out just how much if I could remember where in my old physics text it is.)

Julie Worth said:

... If the shooter is floating around in zero g, firing the gun will tumble him backwards, head over heels. So he’ll have a devil of time getting off another round accurately until he hits a wall and stops tumbling.

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Yeah, unless he's an engineer and realizes that the assembly of him and the gun are in effect a rocket. If he holds the gun precisely in line with his center of mass... Nah. I keep forgetting that he's a maniac.

... A space gun might use bullets that are essentially tiny rockets, in which case the kick-back on the shooter would be minimal. The bullet would bury itself into the thick plastic window, with the hot jet of gasses heating it cherry red, until...blamo! The decompression sucks shooter out the window, while the quick-witted hero slams his faceplate closed and survives.

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These exist, sorta. They're called tracers. A tracer bullet is hollow, contains a phosphorus compound that burns as it flies toward the target. There is some contribution to accelleration, I suppose, but not a lot. Problem with tiny rockets is that they're extremely difficult to control. They could be provided with vanes but those wouldn't be of any benefit in a vacuum. My vote is still for the ray blaster.

As for air pressure having any measurable effect on kickback, nope.

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You're right of course. I was thinking of aerodynamics (necessary to guide the little bullet rockets) and that an non-streamlined object such as our maniac would slow down faster in air than he would in vacuum. But again, unless he has a HUGE gun (and as you point out, holds it off-center), he's not gonna move much from the kickback.

I'm always amazed that the astronauts in the ISS can move with seemingly jerky movements and as rapidly as they do without causing more tumbling to themselves. In the recent shuttle mission, Commander Eileen Collins, while waiting for radio connection to Huston for an interview, was toying with the phone thing. She could make it tumble and roll about readily, but her actions had almost no effect on her. The reason is the difference in mass between the commander and her phone. You pointed that out, too. A bullet has negligible mass compared to the gun or the person holding it. And again, I'm sure this can be worked out, but my conjecture is that firing a gun in zero-G would initiate a tumbling motion to our maniac, but it would be one he could control readily. Especially if he is familiar with low-gravity environments.

Some more thoughts:

The gun in Julie's example is pretty large compared to something like a typical hunting rifle. A 30-30 would fire a bullet weighing closer to 10 grams and a slightly lower velocity, more like 725 m/sec. The kick on that would be a lot less. You can take a look at rifle cartridge specs here, although the mass is in grains and the velocity is in feet per second. I use Online Conversion to put them in more useful units. And there's another one for pistol cartidge specifications. If you're moderately obsessive about getting technical details, these pages can give you some idea of how much a contemporary weapon would knock the shooter backwards in space.

A gun on a spaceship might not be loaded with conventional bullets, for exactly the reason Pthom listed. There are quite a few sorts of bullets in use today for police and home defense units that are designed to break apart on impact. They'll do a number on flesh, but shoot one through a wall and it may not come out with very much killing power on the other side. If you have a military spaceship, chances are soldiers who fight aboard ships would carry guns loaded with similar bullets.

Also, about that window. Even if the window suddenly shatters or pops out, the decompression from normal atmospheric pressure may not be all that explosive. There'd be something of a wind, but if there are straps to grab ahold of, it's certainly possible that some crew members might survive if they were wearing space suits (or could get to an airlock in time).

Firing a gun underwater can have a few other complications. I've heard the noise can permanently damage your hearing if your ears are also underwater.

Julie Worth said:

A space gun might use bullets that are essentially tiny rockets, in which case the kick-back on the shooter would be minimal.

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Such a weapon exists. It was developed in the early 1960s and was manufactured and sold as the Gyrojet pistol. Its rocket bullets reached a speed of Mach 2 just after exiting the perforated barrel and could be fired effectively in water, air, and vacuum conditions. I've heard it has virtually no kick-back. Though it was very quiet, its accuracy was poor and the exhaust could give away your position. Also, at that time the cartridges cost $1.00 apiece.

They later produced a Gyrojet rifle, but by then they'd missed the marketing window as they'd tried to earlier convince the military to switch from the .45 caliber handgun to it.

You might still find information about it on the Internet. It might even still be sold. Okay, here's a link: http://www.securityarms.com/20010315/galleryfiles/1700/1787.htm

A thought occurred to me that this would be an excellent obscure weapon to someday feature in a CSI episode since some of the ammunition now sells for $100 a round. Of course, our assailant would have stolen the gun and ammunition from a bonafide collector, but that would only make discovering the trail all the more interesting since the assailant would have to have enough knowledge about the weapon to deliberately choose it for a murder or possibly be trying to frame the collector (or both).