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Posted: 2/1/2006 8:38:30 AM EDT
Anyone know of a good site that discusses the science behind the way sandbags stop small arms? I know it has a lot to do with energy dispersion and causing the bullet to tumble and fragment. But I need some more information from a physics persepective. It's for a friend who's working on a science project with his daughter, and since I'm the resident "gun guy" he asked me for some help.
Link Posted: 2/1/2006 8:40:06 AM EDT
Old_Painless did a study on his Box-of-Truth site.
Link Posted: 2/1/2006 8:41:41 AM EDT

Originally Posted By VTHOKIESHOOTER:
Old_Painless did a study on his Box-of-Truth site.



www.theboxotruth.com/docs/bot7.htm
Link Posted: 2/1/2006 8:46:22 AM EDT
Link Posted: 2/1/2006 8:58:29 AM EDT
if it was all friction the bullet would not deform, except for some thin shear deformation on the surface.

Some is absorbed by doing work on the bullet. The physical deformation (plastic deformation) of the bullet metal up to the rupture point of the metal. Also some energy is absorbed performing the the physical work on the sand, when some is ground to smaller particles.

In the case of water, its friction drag, plus the pressure drag which is converted to momentum which is transfered to the structure holding the jug/tank.

What are the percent energy splits? Sorry, thats where i start getting paid to answer questions.
Link Posted: 2/1/2006 9:00:47 AM EDT
FRICTION!

Link Posted: 2/1/2006 9:04:16 AM EDT
The sand readily absorbs all the energy in the bullet. It loses interia and momentum almost immediately. An arrow with its long length will actually quite easily go through a sand bag. And that s because there are parts of the arrow shaft still with energy enough to drive it through.
Link Posted: 2/1/2006 9:05:57 AM EDT
I remember reading about this in an Urban Combat FM (can't remember the number) in 1989. According to the FM, it takes .50 BMG and up to bust sandbags.

If you can find the FM, it's highly entertaining-they suggested duct taping a claymore to a wood plank, extending the plank around a corner, and letting her rip!
Link Posted: 2/1/2006 9:08:18 AM EDT

Originally Posted By crurifragium:
I remember reading about this in an Urban Combat FM (can't remember the number) in 1989. According to the FM, it takes .50 BMG and up to bust sandbags.

If you can find the FM, it's highly entertaining-they suggested duct taping a claymore to a wood plank, extending the plank around a corner, and letting her rip!



Hold my beer and watch this!
Link Posted: 2/1/2006 9:09:58 AM EDT
By being in the path of the bullet.
Link Posted: 2/1/2006 9:12:17 AM EDT

Originally Posted By skpp108:
By being in the path of the bullet.



+1
Link Posted: 2/1/2006 9:15:24 AM EDT
[Last Edit: 2/1/2006 9:16:01 AM EDT by Old_Painless]

Originally Posted By Paul:
It disapates the energy as heat just as water does. The energy contained in the moving mass is absorbed and converted to heat which warms the sand bag ever so slightly.

Newton rules!




Paul is correct.

In our experiments, we found that the sand directly in the path of the bullet was converted into a fine powder and got hot enough that even a few minutes later, the heat could be felt by a bare hand.

The rifle bullets seemed to penetrate even less than the slower moving pistol bullets. The rifle bullets were torn into small pieces by the energy conversion.

Sand makes good cover.
Link Posted: 2/1/2006 9:19:48 AM EDT

Originally Posted By ARDOC:
The sand readily absorbs all the energy in the bullet. It loses interia and momentum almost immediately. An arrow with its long length will actually quite easily go through a sand bag. And that s because there are parts of the arrow shaft still with energy enough to drive it through.



I don't think that is quite right.

A rifle bullet has much more energy than an arrow.

If you could keep the bullet traveling "pointy end first", it would penetrate the sandbag.
But since it tumbles, and the profile has much more resistence in the sand, it is slowed much quicker.

Sand backs itself up. I forget what the scientific term is.
It is like silly puddy. Soft and strechy if moved slowly.
More firm if struck suddenly.

You can easily push a bullet through the sand.
But at higher velocities, the sand compresses and is backed by what is behind it.
The force is spread out to an area MUCH larger than that of the bullet.

Keeping the projectile heading straight into the sand, it would push the sand out of the
way, virtually aerodynamically, instead of pushing it forward.
Link Posted: 2/1/2006 9:22:51 AM EDT
I might have got the terminology incorrect but the physics is correct. A 308 bullet will be stopped by a sandbag but a lot archers can penetrate a sandbag with an arrow.

It has to be true. I saw a demonstration of THATS INCREDIBLE as a kid.
Link Posted: 2/1/2006 9:25:12 AM EDT
The term I was thinking of is "shear thickening fluid".

Granted, sand isn't a fluid, but it can be treated as such for what we are talking about.
Link Posted: 2/1/2006 9:26:39 AM EDT

Originally Posted By ARDOC:
I might have got the terminology incorrect but the physics is correct. A 308 bullet will be stopped by a sandbag but a lot archers can penetrate a sandbag with an arrow.

It has to be true. I saw a demonstration of THATS INCREDIBLE as a kid.



If you could somehow manage to stabilize the 308 bullet so it didn't tumble,
it would go right through it too.



Link Posted: 2/1/2006 9:28:43 AM EDT

Originally Posted By ARDOC:
The sand readily absorbs all the energy in the bullet. It loses interia and momentum almost immediately. An arrow with its long length will actually quite easily go through a sand bag. And that s because there are parts of the arrow shaft still with energy enough to drive it through.



I know arrows go through garage doors and power mirrors on cars, not at the same time though.
Link Posted: 2/1/2006 9:29:05 AM EDT

Originally Posted By The_Reaper:

Originally Posted By ARDOC:
I might have got the terminology incorrect but the physics is correct. A 308 bullet will be stopped by a sandbag but a lot archers can penetrate a sandbag with an arrow.

It has to be true. I saw a demonstration of THATS INCREDIBLE as a kid.



If you could somehow manage to stabilize the 308 bullet so it didn't tumble,
it would go right through it too.






No, it would not. Round balls are stopped by sandbags, and they don't "tumble" at all.
Link Posted: 2/1/2006 9:43:36 AM EDT

Originally Posted By The_Reaper:

Originally Posted By ARDOC:
I might have got the terminology incorrect but the physics is correct. A 308 bullet will be stopped by a sandbag but a lot archers can penetrate a sandbag with an arrow.

It has to be true. I saw a demonstration of THATS INCREDIBLE as a kid.



If you could somehow manage to stabilize the 308 bullet so it didn't tumble,
it would go right through it too.




I believe that the issue is that the arrow is moving slowly enough to "push" the sand grains aside instead of crushing them as the bullets do.

In crushing the sand grains, the bullets are torn apart. The arrow just "moves" them aside as it passes through.
Link Posted: 2/1/2006 9:49:07 AM EDT

Originally Posted By Paul:
It disapates the energy as heat just as water does. The energy contained in the moving mass is absorbed and converted to heat which warms the sand bag ever so slightly.




Only Mil spec sand works like that! And Since Clinton banned the sale of Mil stuff to civilians, you have to use playground sand which is half as good.

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