Speed of Objects in Space - Is It the Same as On Earth?

[Mariko]

I remember my Astronomy teacher saying something to the effect that if you were in space and you had a book and you pushed the book it would then travel through space at the same speed and in the same direction constantly for ever unless of course something obstructed it. So say you were in space and some crazy astro guy shoots a gun at you, then that bullet would be just as fast? and as deadly to you as it would on Earth so long as it has the initial force behind it? Or would space somehow slow the bullet down(also I am not being spacific as far as a gun being the initial force because I am unsure a gun would shoot the same way in space)

 

[cepheid]

Why would space slow the bullet down? The whole point your astronomy teacher was making was that almost *nothing* would slow the book down, it would remain traveling at a constant velocity in a straight line unless acted upon by some external force (which is less likely to happen in space). Look up "Newton's First Law of Motion."

As a side note, there is no oxygen in space to ignite the gunpowder, so it wouldn't actually fire. But if it somehow could, then the bullet's motion would be much simpler than it is on earth. In space it would travel in a straight line at a constant speed. On earth, it travels in a parabolic trajectory and aerodynamics come into play.

 

[Labguy]

As a side note, there is no oxygen in space to ignite the gunpowder, so it wouldn't actually fire. But if it somehow could, then the bullet's motion would be much simpler than it is on earth. In space it would travel in a straight line at a constant speed. On earth, it travels in a parabolic trajectory and aerodynamics come into play.

That's absurd. A firearm doesn't need a source of oxygen to fire since the oxygen and propellant are already part of the gunpowder. It would fire just fine in space, and the bullet velocity would actually be higher than on Earth since there is no air in the barrel to be forced out of the way.

 

[SpaceTiger]

Or would space somehow slow the bullet down(also I am not being spacific as far as a gun being the initial force because I am unsure a gun would shoot the same way in space)

If you could get your gun to fire, the primary force on the bullet would be gravity, assuming you were near the sun or earth. Bullets typically go ~1 km/s, while the escape velocity from the solar system at our distance from the sun is ~40 km/s and the escape velocity from the Earth is ~10 km/s near the surface. If you fired it from low-earth orbit, your bullet would likely go into orbit about the earth. There are other forces, like drag and magnetic forces, but they would be very tiny in space.

 

[Integral]

Unless the gun were firmly anchored to something the bullet will have to share the momentum with the man firing the gun. Therefore its initial velocity will be a bit less then that of a bullet fired on earth. This may counter the gain due to a lack of air in the barrel. Once fi

 

[Mariko]

Thanks guys I appreciate your answers.

 

[tony873004]

Unless the gun were firmly anchored to something the bullet will have to share the momentum with the man firing the gun. Therefore its initial velocity will be a bit less then that of a bullet fired on earth. This may counter the gain due to a lack of air in the barrel. Once fi

That's what I was thinking. But then I thought perhaps not because with your feet planted on the ground, you and the Earth become a single object, assuming the recoil force does not exceed the static friction between your feet and the ground. So instead of |P_astronaut| = |P_bullet| you have |P_earth+person|=|P_bullet|. The bullet goes forward as normal, but instead of you going backwards like the astronaut, you alter the rotation of the Earth a very small amount.

And since your frame of reference in the astronaut problem is probably the rest frame of the astronaut before firing the gun, and the frame of reference on Earth is probably Earth, and you just accelerated Earth in a direction opposite the bullet, this tiny insignificant amound would actually add to the bullet's velocity. But in the astronaut example, if the frame of reference were the astronaut, then the bullet would be moving faster than the Earth bullet since the astronaut's momentum, which would equal the Earth's momentum in the Earth version of the problem, would have a much greater velocity.
Just my guess.

 

[cepheid]

Oh well, my mistake then (about the gunpowder). Not absurd, just incorrect.

 

[Labguy]

Oh well, my mistake then (about the gunpowder). Not absurd, just incorrect.

Sorry about that; sometimes I just get excited. It was just incorrect.

Unless the gun were firmly anchored to something the bullet will have to share the momentum with the man firing the gun. Therefore its initial velocity will be a bit less then that of a bullet fired on earth.

In space, the bullet would still be faster. When shooting on Earth, the bullet mass and expanding gasses force the shooter (and gun) back, as recoil, from bullet mass and V, but the bullet is slowed quite a bit from the air in the barrel being greatly compressed and forced into the "outside" air. Almost all resistence to the bullet is from air being compressed. A bit is from the bullet/barrel friction. In space, the bullet compresses no air and does not have to push any air from the barrel into outside air. The shooter and gun will still recoil, but their mass is so great that the V is slow compared to the bullet, either on Earth or in space. At the slow V for the shooter on Earth, air compression is essentially zero at the very low V that the shooter/gun is rocked back, but the bullet still has to do the "air work" routine.

In either condition, the bullet/barrel resistence from friction would be the same total. Bullet V is slower on Earth so friction applies longer, but in space the bullet V is faster (more friction) but remails in the barrel for a shorter time period (less friction).

There is a finite possibility/probability that the paragraphs above are correct...

For your viewing pleasure, I will try to attach a 5-shot target photo that I shot with my Anschutz .22 (German, olympic-style gun). It was only 25 yards, but the center-to center spread was only ~0.06 inches! Usually, 0.25 inches is considered very accurate; I had a lucky day. IF your monitor is set at 1024x768 pixels, the target is actual size.

 

[SpaceTiger]

Unless the gun were firmly anchored to something the bullet will have to share the momentum with the man firing the gun. Therefore its initial velocity will be a bit less then that of a bullet fired on earth. This may counter the gain due to a lack of air in the barrel.

In space, the bullet would still be faster. When shooting on Earth, the bullet mass and expanding gasses force the shooter (and gun) back, as recoil, from bullet mass and V, but the bullet is slowed quite a bit from the air in the barrel being greatly compressed and forced into the "outside" air.

Tsk tsk. Quibbling over such small factors in an astronomy is, I dare say, heresy.

 

[Labguy]

Tsk tsk. Quibbling over such small factors in an astronomy is, I dare say, heresy.

Quibbling? Heresy? This is some serious astronomy stuff we've got going here! When we (humans) get into space to stay our Storm Troopers are going to have to know how to shoot with zero bullet drop and know what recoil effects will apply so they can have at least a chance to defeat the Evil Empire and a bunch of guys whose names all start with "Darth". The fate of the entire human race might depend on what got started on a PF post way back in 2006...

(note: Labguy, get a life.)