Sharp as a Marble - How would this work exactly?

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Guns. Lots of Guns.

Apparently, the Ruskies keep a loaded firearm in their little spaceship. While I doubt there's much of a need for one up there, I don't begrudge them for having it. However, guns and Zero-G don't make a lot of sense for me.

Here on Earth, I use the friction generated by my feet against the ground (which are held together via gravity) to stay in one place when I shoot. The amount of force pushing against my body is not enough to overcome the friction of my soles.

In space, if you shot a gun, wouldn't you also go back at a few hundred feet per second?

Or do I need to brush up on my physics again?

posted @ 2/14/2008 1:46:49 PM

Comments

MadRocketScientist says:

on 2/14/2008 2:12:53 PM

Brush up on your physics. Shooting a gun would propel you backwards, but not by much as the bullet would have to overcome your inertia. If somehow you were able to direct all of the energy of the bullet into you, you might get moving a little bit more.

Remember, the equation is m1v1 = m2v2, or mass x velocity of the first object will equal the mass x velocity of the second.

So a .45 (13 grams) leaves the barrel at 290 m/s so 0.013*290 = 3.77kg.m/s.
A man weighs 85 kg, so 3.77 / 85 = 0.044 m/s of velocity imparted to you

DM says:

on 2/14/2008 2:13:54 PM

You would be pushed back, however not that fast. You must take into account the mass of the bullet vs. the mass of your body+gun.

MadRocketScientist says:

on 2/14/2008 2:17:26 PM

keep in mind if a bullet had the energy to accelerate you to a few hundred feet per second, it would do so INSTANTLY (or at least as fast as the bullet accelerates). That much energy means that shooting a gun at the range would likely break your arms.

MadRocketScientist says:

on 2/14/2008 2:18:53 PM

Which is also why bullet impacts don't send people flying, despite the best attempts at Hollywood to demonstrate otherwise

Gregory Morris says:

on 2/14/2008 2:20:29 PM

And, uh, you'd put a hole in the tiny tincan you are living in, which quite frankly could be worse than anything the gun might be used against. Unless they have special "space bullets". If they do, I want some.

mgroves says:

on 2/14/2008 2:28:56 PM

Didn't you see Moonraker? C'mon!

MadRocketScientist says:

on 2/14/2008 2:40:20 PM

Greg,
Frangible rounds would likely be OK in the ISS, as it has to withstand micro-meteor impacts. And the ISS has repair kits to fix leaks, so while I would hardly recommend casually shooting a handgun up there, one errant round is unlikely to cause explosive decompression (unless it smacks some kind of airlock control, but methinks that is more Hollywood type incidence).

BobG says:

on 2/14/2008 2:42:22 PM

I'd worry more about the bullet bouncing around and hitting something important----like me.

Robb Allen says:

on 2/14/2008 3:15:10 PM

Dammit, I wrote that wrong. I know you wouldn't go back in the hundreds of feet per second, I meant to say wouldn't the X foot pounds at Y FPS have a rather large effect on you?

When I shoot, it's enough to push my arms up. I wondered in 0G if that would translate into a nice little shove.

MadRocketScientist says:

on 2/14/2008 3:23:55 PM

As I showed up above, you'd get pushed back 0.044 m/s, or .144357 ft/sec, or 0.1 mph.

It ain't much, but if you had a mini-gun and 10000 rounds of ammo, I bet you could get moving at a pretty good clip.

Robb Allen says:

on 2/14/2008 3:32:13 PM

My Off the Top of the Head calculations were basically "Lessee... 800 foot pounds muzzle velocity, 200 pound guy, 800/200 = 4. So 4 something."

MadRocketScientist says:

on 2/14/2008 4:11:16 PM

Robb,

You are confusing energy and momentum. A .45 ACP round leaves the barrel with about 370 ft-lbf (foot-pound force) of kinetic energy, but only 3.77 kg-m/s of momentum.

Kinetic Energy = 1/2 x mass x velocity x velocity

Momentum = mass x velocity

Make sense, or do I need to give a full lesson on kinematics?

straightarrow says:

on 2/14/2008 6:44:18 PM

kinetics? Wasn't that invented by L. Ron Hubbard?

straightarrow says:

on 2/14/2008 6:58:58 PM

Rob I put that comment on the wrong post, could I get you to move it to the post about gunlocks?

Billll says:

on 2/14/2008 8:16:14 PM

I understand the gun is a holdover from the Russkie space program. They land their astronauts on land, in Siberia, and getting to the landing site is not as easy as the ocean-based landings the US used, pre-shuttle. The gun is a survival tool for an astronaut lost in the Siberian woods.

There is a scene in the book "Ragamuffin" in which a minigun is used as a propulsive device, and a submachine gun is used for steering control.

Justin Buist says:

on 2/14/2008 11:41:28 PM