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InvisibleDoc_T
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Re: so in theory in space....... [Re: imachavel]
    #14831350 - 07/27/11 03:41 AM (12 years, 9 months ago)

I'm up early. :frown:

There will be gravity in space, and also a certain amount of material, even between galaxies.
But yeah, a bullet would travel for millions of years until it hit something.


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Re: so in theory in space....... [Re: Doc_T]
    #14831389 - 07/27/11 03:58 AM (12 years, 9 months ago)

it should. check this out:



I wish he threw those balls really hard I'd like to see how they bounce around :lol:


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Re: so in theory in space....... [Re: DieCommie]
    #14831411 - 07/27/11 04:18 AM (12 years, 9 months ago)

> 1e-3 = 1x10-3.

Yes, that is what I meant. 

> The gas rushing out would continue at least a second though it would drop off rapidly.

We are talking about acceleration (and equilibrium), not 'continuing'.  I made a rough estimate given the mass of the gas versus the mass of the bullet, the freedom of the gas to diffuse, and an estimated speed of the bullet.  If the bullet has a velocity of 1000m/s, then I don't find it unreasonable that the gasses would continue to add to the bullets acceleration for the first meter after leaving the barrel, or around 1/1000th of a second.  After that, the force that the gas can exert on the bullet is going to be negligible, thus the acceleration is going to be zero (negligible).


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InvisibleStonehenge
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Re: so in theory in space....... [Re: Seuss]
    #14832502 - 07/27/11 11:09 AM (12 years, 9 months ago)

Glad to see you back in the thread, seuss. This has really revived interest in science so no matter what we conclude, it contributed something.

Sorry, but i'm going to have to be picky here and pick apart many of your statements. I realize you were speaking in general so maybe that isn't totally fair but we want to find the truth, not generalities.

>1/1000th of a second.  After that, the force that the gas can exert on the bullet is going to be negligible, thus the acceleration is going to be zero (negligible).

First of all, negligible is not the same as zero. We are talking about whether there will be any change in the bullets velocity, either positive or negative acceleration. There is not threshold below which we can ignore forces and say they are "negligible" A force that exerted one billionth of a gram of force on the bullet would have a major impact on it's trajectory over a long distance. We are talking about space, not firing a bullet into a target a short distance away. Therefore even slight forces must be acknowledged.

The bullets mass is not "many orders of magnitude" greater than that of the gas. The mass of the gas is equal to the mass of the propellant which is perhaps one order of magnitude less than that of the bullet. As long as the hot gas pushed into the bullet, a slight acceleration would result. It would fall to zero when no more molecules of gas hit the bullet.

DieCommie, there is a reason the bullet has an angular momentum put on it by rifling in the barrel. It is precisely to prevent tumbling, not to cause it. This is due to the gyroscopic effect.

http://en.wikipedia.org/wiki/Gyroscope

Far from causing tumbling, spin tends to resist tumbling when traveling through the atmosphere. In space where there is no outside force which could cause tumbling, the spin prevails. The energy in the rotation of any object resists a change in orientation such as spinning end over end. Without a strong outside force, the bullet will continue to spin and to travel with the nose forward.

As for the color affecting the amount of heat absorbed or given off, pretty much anyone knows that dark colors absorb heat much faster than light ones. Try wearing black clothes out in the sun on a hot day if you doubt it. As for dark colors radiating more heat, why do you suppose parkas and other cold weather gear is usually white and never black? Here is an excerpt from a physics professor

http://www.newton.dep.anl.gov/askasci/phy00/phy00156.htm

>The darker the object, the better its emission of heat because it is a better absorber of light.

I hope this settles a few of the points. There are still more things to discuss, ever slighter forces which would act on the bullet. Eventually it would hit atoms or molecules of hydrogen which exist between galaxies.


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OfflineSeussA
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Re: so in theory in space....... [Re: Stonehenge]
    #14832769 - 07/27/11 12:13 PM (12 years, 9 months ago)

> First of all, negligible is not the same as zero.

I use the words interchangeably in the context we are discussing.  As an example, there are a few particles of gas (hanging out in space) that the bullet is going to hit as well that are going to slow the bullet down a little bit as energy is transferred from the bullet to these few particles of gas.  However, the net result is negligible... as in literally unmeasurable... thus given the magnitude of the numbers we are working with, the effect is insignificant.

> The bullets mass is not "many orders of magnitude" greater than that of the gas. The mass of the gas is equal to the mass of the propellant which is perhaps one order of magnitude less than that of the bullet.

I'm talking specifically about the mass of the few stray gas particles that are going to strike the bullet some time after the bullet has left the barrel.  Once free from the confines of the barrel, most of the gas is going to quickly dissipate.  Even in the barrel, you might have 30 grains of propellant vs 150 grains of bullet.  That is still over an order of magnitude difference.

A good thought experiment... pretend that the gas is a single solid object behind the bullet (such as another bullet) and imagine what happens when that single large object pushes the bullet out of the barrel.  If the single large object continues to accelerate the bullet, what force is acting on the single large object?


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InvisibleStonehenge
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Re: so in theory in space....... [Re: Seuss]
    #14833139 - 07/27/11 01:18 PM (12 years, 9 months ago)

OK, you clarified what you meant by several things. If you are referring to the time at which the gas has diffused down to a few molecules, it's not too out of line to call it negligible. However, over a long distance even the solar wind will make a difference.

I also mentioned the stray atoms of gas the bullet would encounter. It's my understanding that the density of the galactic nebulae is about 1 atom of hydrogen per cubic meter of space. About what we would call an extremely hard vacuum. Empty space would be expected to have a much lower density.

>Once free from the confines of the barrel, most of the gas is going to quickly dissipate.

Sure, but it will follow a cone shape as it's tendency to spread out interacts with it's forward velocity. .001 seconds later i don't think we will be down to a molecule or two.

>Even in the barrel, you might have 30 grains of propellant vs 150 grains of bullet.  That is still over an order of magnitude difference.

Correct me if i'm wrong but i thought an order of magnitude was 10x?

>A good thought experiment... pretend that the gas is a single solid object behind the bullet (such as another bullet) and imagine what happens when that single large object pushes the bullet out of the barrel.  If the single large object continues to accelerate the bullet, what force is acting on the single large object?

Then we would have a totally different situation. Rather than the propellant being a solid object that got it's momentum from something else, it is a dynamic expanding gas. It is pushing on the gun at the same rate it pushes on the bullet so the gun is going to accelerate backward after firing. The gas continues to expand and even in empty space, a gas can expand and push in all directions. I grant you it would be quickly dissipated and we can discuss what amount of time it would take to get down to a molecule or two striking the bullet. It's the equivalent of an explosion in space and the force is generated by that explosion.


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Re: so in theory in space....... [Re: Stonehenge]
    #14833201 - 07/27/11 01:30 PM (12 years, 9 months ago)

Quote:

Stonehenge said:


First of all, negligible is not the same as zero. We are talking about whether there will be any change in the bullets velocity, either positive or negative acceleration. There is not threshold below which we can ignore forces and say they are "negligible" A force that exerted one billionth of a gram of force on the bullet would have a major impact on it's trajectory over a long distance. We are talking about space, not firing a bullet into a target a short distance away. Therefore even slight forces must be acknowledged.




You use relative terms such as "major" impact and "long" distance versus "short" distance.  To speak of such relative qualities and then conclude there is no negligible factor, such as a nanogram, is silly.

Quote:



DieCommie, there is a reason the bullet has an angular momentum put on it by rifling in the barrel. It is precisely to prevent tumbling, not to cause it. This is due to the gyroscopic effect.




?  What's your point?  The angular momentum is conserved when the bullet changes from spinning on its long axis to its short axis.  As for the intent of using rifling, unfortunately the universe isn't too concerned with our intentions.


Quote:


Far from causing tumbling, spin tends to resist tumbling when traveling through the atmosphere. In space where there is no outside force which could cause tumbling, the spin prevails.




Spin is not conserved, angular momentum is.  You have not established that there is any violation of the conservation of momentum for a bullet to transition to turning about its short axis, so there is no reasoned objection.  I've worked out above a very simple example showing why the tumbling bullet has less kinetic energy for the same angular momentum, and you've taken no issue with this nor provided any other objection.

As for the relatively slight forces on the bullet compared to earth, you don't say what the relevance of this comparison is, and I don't see it, either.  The discussion is over what the bullet will tend to do over time, what the lower energy state is, and hence I don't see how a discussion of the relative time it would take could be contradictory in the slightest.

Quote:

The energy in the rotation of any object resists a change in orientation such as spinning end over end. Without a strong outside force, the bullet will continue to spin and to travel with the nose forward.




Again, what does the strength of the force have to do with a discussion of the bullet's tendancy over time? 

Quote:

As for the color affecting the amount of heat absorbed or given off, pretty much anyone knows that dark colors absorb heat much faster than light ones.




So what?

Quote:

As for dark colors radiating more heat, why do you suppose parkas and other cold weather gear is usually white and never black? Here is an excerpt from a physics professor




Ignoring style, I would imagine its to reflect radiation.  As the flux through the parka from the body is surely greater than that from the environment, this makes sense.  What does this have to do with black body radiation?  The whole point is to ignore reflection.



Quote:

>The darker the object, the better its emission of heat because it is a better absorber of light.




So what?  The whole point is the emissions due to the object's own thermal energy, temperature.  Considering a case where the object is heated by incoming radiation supposes an additional energy source, which is not what the black body case is examining.

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Re: so in theory in space....... [Re: Seuss]
    #14833213 - 07/27/11 01:32 PM (12 years, 9 months ago)

.

Edited by DieCommie (11/10/16 07:22 PM)

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InvisibleStonehenge
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Re: so in theory in space....... [Re: DieCommie]
    #14833375 - 07/27/11 02:06 PM (12 years, 9 months ago)

DC and john, i'm sorry you don't understand the physics of a gyroscope or how it affects the trajectory of a bullet. I provided a link but it seems to have been ignored. You can say "so what" as many times as you want but not only physicists but people in the field have proven what i said to be true.

>Spin is not conserved, angular momentum is

Spin IS angular momentum. In order for the bullet to tumble end over end, it must overcome not only inertia but also the spin put on by the barrel. You and dc seem to think the angular momentum will translate into end over end tumbling due to your interpretation of something or other. You have shown nothing to demonstrate this phenomenon or given any links. You don't see seuss agreeing with that part. He knows better.

>Yes, anyone knows that dark clothes absorb heat faster.  But they do not give off heat faster.

Did you follow that link? You are saying you know more about the subject than a physics professor? Where is your proof of that? I gave my evidence, where is yours?


--------------------
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Re: so in theory in space....... [Re: Stonehenge]
    #14833505 - 07/27/11 02:33 PM (12 years, 9 months ago)

Quote:

Stonehenge said:
DC and john, i'm sorry you don't understand the physics of a gyroscope or how it affects the trajectory of a bullet. I provided a link but it seems to have been ignored. You can say "so what" as many times as you want but not only physicists but people in the field have proven what i said to be true.




You don't seem to understand that a bullet is not a gyroscope.

Quote:


Spin IS angular momentum.




"Spin", as you are using it, is a loose, ambiguous shorthand.  Angular momentum on the other hand, is well-defined.

Quote:

In order for the bullet to tumble end over end, it must overcome not only inertia but also the spin put on by the barrel.




I don't understand this sentence.  The angular momentum of the bullet remains the same when the bullet transitions from minor axis to major axis rotation.  The energy of the bullet also decreases slightly in the process making the process a favorable one. 

Quote:


The bullet You and dc seem to think the angular momentum will translate into end over end tumbling due to your interpretation understanding of something or other basic physics.




Fixed it for you.

Quote:

You have shown nothing to demonstrate this phenomenon or given any links.




Actually, they both provided explanations.  Two entirely different explanations that arrived at the same result.

Quote:


Did you follow that link? You are saying you know more about the subject than a physics professor? Where is your proof of that? I gave my evidence, where is yours?




Where in that link does anyone say that dark colors emit heat at a higher rate than light colors?  What I see is a statement that darker colors absorb more visible light which is turned into heat internally and then re-emitted.  So, subject to an external source of radiation, darker objects convert light into heat more rapidly.  When dealing with a blackbody, however, we aren't considering external light sources so this is completely irrelevant.

[Edit:  Last paragraph is written poorly, reader beware.  :wink:]

Edited by ChuangTzu (07/27/11 07:47 PM)

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Re: so in theory in space....... [Re: ChuangTzu]
    #14833895 - 07/27/11 03:57 PM (12 years, 9 months ago)

You guys aught to go explain to the gun makers and shooters that they are all wrong about putting spin on a bullet. They think it prevents tumbling, but you folks seem to know better than me and better than them that it's the opposite. You have explained that it promotes tumbling. I'm sure they will smack themselves in the face and say "of course, we've been doing it backwards all this time"

Hurry up before someone else tells them and maybe you can get a nobel prize or something?

And then explain to the parka makers that black does not give off any more heat than white and since it absorbs heat better, they should make winter clothing in black only. They will do the forehead smack and thank you for your help.

I'm convinced, now go out and explain it to those fools who still think the other way.

Seuss, you are convinced too aren't you? How could you doubt all that sincerity?


--------------------
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InvisibleDieCommie

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Re: so in theory in space....... [Re: Stonehenge]
    #14833911 - 07/27/11 04:00 PM (12 years, 9 months ago)

Quote:

I'm convinced, now go out and explain it to those fools who still think the other way.




Thats my job.  :grin:

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Re: so in theory in space....... [Re: DieCommie]
    #14833924 - 07/27/11 04:03 PM (12 years, 9 months ago)

Quote:

DieCommie said:
Quote:

I'm convinced, now go out and explain it to those fools who still think the other way.




Thats my job.  :grin:




Physics is tough like that. :sadyes:


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Re: so in theory in space....... [Re: Doc_T]
    #14834014 - 07/27/11 04:27 PM (12 years, 9 months ago)

hard core :shrug:


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Re: so in theory in space....... [Re: Stonehenge]
    #14834291 - 07/27/11 05:26 PM (12 years, 9 months ago)

Quote:

Stonehenge said:
You guys aught to go explain to the gun makers and shooters that they are all wrong about putting spin on a bullet. They think it prevents tumbling, but you folks seem to know better than me and better than them that it's the opposite. You have explained that it promotes tumbling. I'm sure they will smack themselves in the face and say "of course, we've been doing it backwards all this time"




The physics of a bullet in an atmosphere over the course of its couple second flight time are in no way representative of the physics of a bullet in free space over the course of its multi-billion year trajectory.

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Re: so in theory in space....... [Re: Stonehenge]
    #14834420 - 07/27/11 05:51 PM (12 years, 9 months ago)

.

Edited by DieCommie (11/14/16 11:36 AM)

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Re: so in theory in space....... [Re: DieCommie]
    #14834516 - 07/27/11 06:15 PM (12 years, 9 months ago)

:thumbup:

Check out the video on the about half way down the page on the right called "6-Minute video of Astronaut Owen Garriott demonstrating the changing polhode paths of a partially-full drink bottle rotating in zero G aboard the NASA Skylab".

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Re: so in theory in space....... [Re: ChuangTzu]
    #14834757 - 07/27/11 07:03 PM (12 years, 9 months ago)

That is very interesting about polhode motion. This seems to be related to changes in torque experienced by the body. In this case a bullet. It would seem likely that some tiny wobble would be present. However, for this to progress to tumbling end over end would seem to require either outside interference or a liquid or elastic center which causes losses in torque. Where would this come from?

>It is important to note that these changes in the orientation of the body as it spins may not be due to external torques, but rather result from energy dissipated internally as the body is spinning. Even if angular momentum is conserved (no external torques), internal energy can be dissipated during rotation if the body is not perfectly rigid

Since we have no external influences that would slow it's rotation, the fact it is reasonably rigid should seem to minimize any polhode motion showing up. In the near zero temps of outer space, this would be even more so. The earth having an elastic core has a slight polhode motion. The earth is billions of years old. How many billions of years would it take for a detectable motion to be seen in the bullet? However, i commend diecommie for coming up with an interesting effect even if it does not cause the bullet to tumble end over end any time soon. Maybe after a trillion years? It's possible.

Any scientific stuff to back up the theory that black does not give off heat faster than white?


--------------------
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Re: so in theory in space....... [Re: Stonehenge]
    #14834951 - 07/27/11 07:37 PM (12 years, 9 months ago)

Quote:

Stonehenge said:
That is very interesting about polhode motion. This seems to be related to changes in torque experienced by the body.




What do you mean by changes in torque?  Our analysis doesn't include any external torques.

Quote:

In this case a bullet. It would seem likely that some tiny wobble would be present. However, for this to progress to tumbling end over end would seem to require either outside interference or a liquid or elastic center which causes losses in torque. Where would this come from?




A bullet is not a rigid body.  In fact, it is less rigid than the quartz spheres in the article DieCommie linked to...

Quote:


Since we have no external influences that would slow it's rotation, the fact it is reasonably rigid should seem to minimize any polhode motion showing up.




Minimize?  What do you mean?  Making the bullet out of the most rigid material would "minimize" the rate at which this phenomenon occurs, but it would still occur.  Lead is nowhere near the most rigid material known--it's actually quite elastic.  Quartz is very, very rigid and still showed this effect very rapidly in the experiment talked about in DieCommie's link.  I would estimate that the bullet would have completely shifted its axis of rotation within minutes, rather than a trillion years as you assert.

Quote:

In the near zero temps of outer space, this would be even more so. The earth having an elastic core has a slight polhode motion.  The earth is billions of years old. How many billions of years would it take for a detectable motion to be seen in the bullet?




A tiny fraction of a billion.

Quote:

However, i commend diecommie for coming up with an interesting effect even if it does not cause the bullet to tumble end over end any time soon. Maybe after a trillion years? It's possible.




Why don't you calculate it?

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Re: so in theory in space....... [Re: DieCommie]
    #14835939 - 07/27/11 10:47 PM (12 years, 9 months ago)

Quote:

DieCommie said:
Here you go stonehenge,  http://einstein.stanford.edu/highlights/hl_polhode_story.html

It has einstein and stanford in the name so maybe you will take it more seriously than a guy named diecommie.  I like how they articulate the difference between an unstable axis of rotation and moving into a high moment of inertia configuration after losing energy.

Quote:

Every solid body inherently has three principal axes of symmetry through its center of mass, and each of these axes has a corresponding moment of inertia. The moment of inertia about an axis is a measurement of how difficult it is to accelerate the body about that axis. The closer the concentration of mass to the axis, the smaller the torque required to get it spinning at the same rate about that axis.

The shortest axis of symmetry (i.e. the axis through the center of the wood block from front to back) has the largest or maximum moment of inertia. If energy is dissipated while an object is rotating, this will cause the polhode motion about this maximum axis of inertia to damp out or stabilize, with the polhode path becoming a smaller and smaller ellipse or circle, closing in on the axis.

The intermediate axis of symmetry (e.g. the axis through the center of the block of wood, from side to side) corresponds to an intermediate moment of inertia that lies between the minimum and maximum moments of inertia. A body is never stable when spinning about this axis, and dissipated energy will cause the polhode to start migrating to the object’s axis of maximum inertia. The transition point between two stable axes of rotation is called the “separatrix,” along which the angular velocity passes through the axis of intermediate inertia.

The longest axis of symmetry (i.e. the axis through the center the wood block, from top to bottom) corresponds to the smallest or minimum moment of inertia. Rotation about this axis is also stable, but given enough time, such as a body rotating in zero g, any perturbations due to energy dissipation or torques would cause the polhode path to expand, in larger and larger ellipses or circles, and eventually migrate through the separatrix and its axis of intermediate inertia to its axis of maximum inertia.







btw whatever happened to those funny celebrity comics you used to make?? :lol:


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