marine6680
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From a strictly scientific point yes...
Layman's terms... they can make things more relatable.
Layman's terms... they can make things more relatable.
physics question about bullets
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zombietactics
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I know this from actual lab work:
If you take two spheres of identical dimension and exterior construction, but fill them with differing amounts of lead (so that they have a difference in mass)
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And then either drop or launch both spheres into a consistent test medium at differing speeds so that they each generate equal KE.
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They penetrate the test media to identical depths.
If you take two spheres of identical dimension and exterior construction, but fill them with differing amounts of lead (so that they have a difference in mass)
...
And then either drop or launch both spheres into a consistent test medium at differing speeds so that they each generate equal KE.
...
They penetrate the test media to identical depths.
marine6680
New member
Zombie... you are confusing principles and scenarios that are unrelated... so for removed that they are meaningless.
You may get the math and understand the principles... but are not applying them correctly.
In that example... Speed has increased so much... a thousand fold... that is has no relevancy to the topic at hand...
You (a human) are physically incapable of exerting the force required to stop the plate in the time frame required. The speed is too great... in fact, many objects could not exert the force required in the time required.
If an object can not provide a force large enough quickly enough, you will not stop the projectile.
The time frame is important.
Energy is not a vector... Momentum which is a vector quantity requires a force to oppose it. As was said, force is applied over time.
Since force is applied over time, and momentum is a vector, time equals distance traveled along that vector. The longer a given magnitude of force takes to stop an object, means more time, and therefore more distance.
Bullets at best may double in speed from the heaviest to the lightest examples in a given caliber. I.E. 9mm 147gr at just under 1000fps and 50gr at around 2000fps.
Also size...
Expand that plate to the frontal area of the truck, and the forces required are dissipated over a larger area, meaning a wall that would not stop the plate at normal size may stop it when expanded.
A small object applies force on a smaller area, and that smaller area is the only counter force.
Its why nails are pointed... Hit a bit of wood with a hammer and it makes a dent... hit a nail with the same hammer and at the same speed, and the nail is driven into the wood.
To say that frontal area and sectional density do not govern penetration is false... against the math, science, and reality.
Frontal area dictates how concentrated the applied force and energy are at the leading point of contact.
(once an object strikes another, momentum is converted to force, due to speed change over a time)
Sectional density determines how concentrated the momentum and inertia are.
Like the differences between a 1lb sphere that is a foot in diameter, and a 1lb sphere that is 2 inches in diameter.
We all know from practical experience that the smaller object will penetrate/pass through another at lower speeds than the larger one.
When it comes to bullets... numerous tests have born these things out.
Heavy bullets out penetrate lighter faster bullets of the same caliber.
For your second example... spheres behave differently than bullets...
Also, unless you are launching those spheres are high speed, the effects are going to be different.
Dropping a small sphere from a few feet into a container of viscous fluid... is not the same as a fired bullet.
At the increased speeds of a bullet, other factors will come into play that affect things much more than at slower speeds.
There is limited testing on FMJ overall penetration, its very deep in test gel, and most don't bother. So, you may be correct for FMJ... I have no direct testing to show on those. Though I feel there will be a favoring of heavier rounds in penetration there as well.
Another thing to think on... The lighter faster bullet is more likely to be altered structurally... maybe even catastrophically. Rifle rounds in water are an example.
That affects how the forces acting to stop the projectile are applied.
Hollow point or expanding rounds going faster deform faster and increase the forces trying to stop it faster as well.
You may get the math and understand the principles... but are not applying them correctly.
In that example... Speed has increased so much... a thousand fold... that is has no relevancy to the topic at hand...
You (a human) are physically incapable of exerting the force required to stop the plate in the time frame required. The speed is too great... in fact, many objects could not exert the force required in the time required.
If an object can not provide a force large enough quickly enough, you will not stop the projectile.
The time frame is important.
Energy is not a vector... Momentum which is a vector quantity requires a force to oppose it. As was said, force is applied over time.
Since force is applied over time, and momentum is a vector, time equals distance traveled along that vector. The longer a given magnitude of force takes to stop an object, means more time, and therefore more distance.
Bullets at best may double in speed from the heaviest to the lightest examples in a given caliber. I.E. 9mm 147gr at just under 1000fps and 50gr at around 2000fps.
Also size...
Expand that plate to the frontal area of the truck, and the forces required are dissipated over a larger area, meaning a wall that would not stop the plate at normal size may stop it when expanded.
A small object applies force on a smaller area, and that smaller area is the only counter force.
Its why nails are pointed... Hit a bit of wood with a hammer and it makes a dent... hit a nail with the same hammer and at the same speed, and the nail is driven into the wood.
To say that frontal area and sectional density do not govern penetration is false... against the math, science, and reality.
Frontal area dictates how concentrated the applied force and energy are at the leading point of contact.
(once an object strikes another, momentum is converted to force, due to speed change over a time)
Sectional density determines how concentrated the momentum and inertia are.
Like the differences between a 1lb sphere that is a foot in diameter, and a 1lb sphere that is 2 inches in diameter.
We all know from practical experience that the smaller object will penetrate/pass through another at lower speeds than the larger one.
When it comes to bullets... numerous tests have born these things out.
Heavy bullets out penetrate lighter faster bullets of the same caliber.
For your second example... spheres behave differently than bullets...
Also, unless you are launching those spheres are high speed, the effects are going to be different.
Dropping a small sphere from a few feet into a container of viscous fluid... is not the same as a fired bullet.
At the increased speeds of a bullet, other factors will come into play that affect things much more than at slower speeds.
There is limited testing on FMJ overall penetration, its very deep in test gel, and most don't bother. So, you may be correct for FMJ... I have no direct testing to show on those. Though I feel there will be a favoring of heavier rounds in penetration there as well.
Another thing to think on... The lighter faster bullet is more likely to be altered structurally... maybe even catastrophically. Rifle rounds in water are an example.
That affects how the forces acting to stop the projectile are applied.
Hollow point or expanding rounds going faster deform faster and increase the forces trying to stop it faster as well.
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zombietactics
New member
I have looked for ... and have yet to find ... a single experimental instance demonstrating this case, where KE was consistent between the compared rounds. Nor is there any physics-grounded math describing this effect.
I have found cases where people compare faster/lighter vs. heavier/slower ... but of different size or shape (i.e. different calibers, etc.) ... but that's not the same argument at all ... KE would have to be the same in order derive differing momentum, and that's never the comparison made.
Again, I've done the lab work under controlled conditions. The results equal what is predicted by established physical models.
At typical handgun velocities, describe to me the established physical principle(s) by which they behave differently.
Brian Pfleuger
Moderator Emeritus
Your example has nothing whatsoever to do with penetration.
Kinetic energy is the ENERGY available to do work. It can bend steel, melt lead, break things, etc. It is NOT what keeps an object moving.
Momentum keeps an object moving. Keeping an object moving is what is responsible for penetration. Momentum (and sectional density) are responsible for penetration.
My guess is that you're doing this experiment with momentum levels so low that the differences are irrelevant. Without the details, it's impossible to say.
Of course there is. It's called "Conservation of Momentum".
Kinetic energy is not necessarily conserved, in the form of kinetic energy. It is transformed into other forms of energy, like heat.
We just had a nearly identical discussion on this topic...
http://thefiringline.com/forums/showthread.php?t=544160&page=2
I'm going to see if our resident physics PhD will check in on this thread... zombietactics can argue with him.
I did a simple experiment in the kitchen to see if I was wrong. Please understand this is not exact or extensive measuring just by eyeballing.
I took 40" of 1/2"pvc cut a slot @25.06" and another @36" from the bottom.
the slots are for holding the bullets for dropping. How did I get the number.
Using a 110 grn bullet and a 158 grn bullet same diameter same manfacturer xtp hornady.
110grn x 36"= 3960 energy
158grn x 25.06"=3960 energy
By dropping the 110grn bullet from 36" and the 158 from"25.06 into vaseline.
penetration looks the same. Here are the pictures.
I took 40" of 1/2"pvc cut a slot @25.06" and another @36" from the bottom.
the slots are for holding the bullets for dropping. How did I get the number.
Using a 110 grn bullet and a 158 grn bullet same diameter same manfacturer xtp hornady.
110grn x 36"= 3960 energy
158grn x 25.06"=3960 energy
By dropping the 110grn bullet from 36" and the 158 from"25.06 into vaseline.
penetration looks the same. Here are the pictures.
zombietactics
New member
That would be helpful.
We're also forgetting to factor in BC. The heavier bullet will tend to have a higher ballistic coefficient so it will not lose it's velocity as quickly compared to a lighter bullet of similar shape, and equal caliber.
What that means is, you could start with two loads at equal muzzle energies, but the lighter bullet is going to lose it's energy more quickly as it travels through the air, compared to the heavy one.
As far as penetration usually the heavier one because more momentum means more inertia, which means harder to stop.
It's like trying to catch a baseball at 100mph (146.6fps) (104ft-lbs) versus catching a 8lb bowling ball at 19.7mph (29fps) (105ft/lbs)
Even though the bowling ball is significantly slower, it has so much mass and momentum that it'd be the harder one to stop.
What that means is, you could start with two loads at equal muzzle energies, but the lighter bullet is going to lose it's energy more quickly as it travels through the air, compared to the heavy one.
As far as penetration usually the heavier one because more momentum means more inertia, which means harder to stop.
It's like trying to catch a baseball at 100mph (146.6fps) (104ft-lbs) versus catching a 8lb bowling ball at 19.7mph (29fps) (105ft/lbs)
Even though the bowling ball is significantly slower, it has so much mass and momentum that it'd be the harder one to stop.
zombietactics
New member
Well, there are lots of simple experiments from which you'll derive the same results. One of my favorites is dropping two weights (heavier vs. lighter) on top of nails from varying distances to see how far the nail is driven (or 'penetrates')
Amazingly, it is always identical KE which produces identical penetration.
I'd like to see the experiments demonstrating the opposing hypothesis, or at least falsifying this one.
marine6680
New member
Note I added a few things to my above post... I do that at times, rather than add another reply.
My last physics course in college was 13 years ago... but I don't think I am completely off base here.
The magnitude of the forces increase dramatically between objects going a few feet per second to ones going 1000fps...
Forces that had little bearing at slow speeds become relevant at high speeds.
But the differences in speeds between two bullets is not as extreme. The ratios are smaller.
Therefore the difference in effects of other forces are also smaller.
Most tests you refer to are of different kinetic energy levels... often times the lighter faster bullet has an energy advantage.
Ballistic coefficient... it was mentioned.
Yes, BC is basically the determining factor for FMJ bullets. While the affects of traveling through air and a viscous semi solid are different... BC will favor the heavier round, in air and in test media.
My last physics course in college was 13 years ago... but I don't think I am completely off base here.
The magnitude of the forces increase dramatically between objects going a few feet per second to ones going 1000fps...
Forces that had little bearing at slow speeds become relevant at high speeds.
But the differences in speeds between two bullets is not as extreme. The ratios are smaller.
Therefore the difference in effects of other forces are also smaller.
Most tests you refer to are of different kinetic energy levels... often times the lighter faster bullet has an energy advantage.
Ballistic coefficient... it was mentioned.
Yes, BC is basically the determining factor for FMJ bullets. While the affects of traveling through air and a viscous semi solid are different... BC will favor the heavier round, in air and in test media.
marine6680
New member
Don't forget the testing medium reacts very differently when impacted at a few fps and when impacted at 1000fps.
And that difference can not be ignored.
And that difference can not be ignored.
Brian Pfleuger
Moderator Emeritus
Ok... Well... Our resident PhD says I'm doing fine and wonders aloud why I aggravate myself with these arguments.
He and I therefore decline to participate farther. However, here's a paper written by a Dr Ashby regarding arrow penetration. He agrees with me, BTW.
http://cpw.state.co.us/Documents/Hunting/EHU/Momentum-KineticEnergy-ArrowPenetration.pdf
I'm done here.
He and I therefore decline to participate farther. However, here's a paper written by a Dr Ashby regarding arrow penetration. He agrees with me, BTW.
http://cpw.state.co.us/Documents/Hunting/EHU/Momentum-KineticEnergy-ArrowPenetration.pdf
I'm done here.
marine6680
New member
Similar profile bullets?
The problem with water jugs is the fact there are several, and no way to know exactly where a bullet stops at.
Also who is the resident PhD?
The problem with water jugs is the fact there are several, and no way to know exactly where a bullet stops at.
Also who is the resident PhD?
zombietactics
New member
Oh good lord ... people are still passing this around? Well, I suppose if they still quote Marshall/Sanow, I shouldn't be surprised. How long before someone cites "noted forensic pathologist Julian Hatcher" or John Taylor's "knockout formula"?
Ashby isn't a physicist, BTW ... and his "paper" was declined publication by several scientific journals as nonsense.
The single-to-noise ratio on this forum is out-of-hand.
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green_MTman
New member
the bullet with the higher sectional density with penitrate best if it has enough velocity.if the bullet with the higher sectional density has velocity reasonable close the the bullet with the lower sectional density,it will have better penitration.
the good news is that you only need a sectional density of 260 to penitrate any big game in north america,so after that velocity wins
the good news is that you only need a sectional density of 260 to penitrate any big game in north america,so after that velocity wins
marine6680
New member
Reading that paper makes me appreciate SI units even more.
marine6680
New member
Real world results do show a favoring of heavier bullets with regards to penetration.
Ballistics coefficients bear this out as well. Higher BC can better retain energy and momentum down range.
Air and ballistics gel are different densities and viscosity, but effects can be looked at in similar ways. So long as the velocity is not so high that structural integrity is affected... like rifle rounds in water compared to pistol rounds.
There are many variables, and energy is not the only factor to penetration.
Once again, I can point to rifle rounds in water. Much higher energy levels, little penetration.
Heavy hard cast bullets are popular for handgun hunting in tough skinned animals.
Pure science must be backed up with experimentation... and in bullet testing, heavy bullets win.
Thinking on the Spheres of equal size but different weight tests mentioned...
I would bet that drag forces come into play more than with bullets.
The test with bullets into petroleum jelly... Bullets do not fall point first. They fall sideways or maybe base first.
Also at slow speeds the drag forces will be more of a factor, and the heavier is likely to have more, due to its larger size due to added length.
Cavitation effects will play a role in the high speeds of a bullet into a vicious semi solid as well.
A round with higher velocity will cause greater cavitation, resulting from a greater transfer, meaning loss, of energy.
The higher energy and velocity are working against the bullet in regards to penetration.
Ballistics coefficients bear this out as well. Higher BC can better retain energy and momentum down range.
Air and ballistics gel are different densities and viscosity, but effects can be looked at in similar ways. So long as the velocity is not so high that structural integrity is affected... like rifle rounds in water compared to pistol rounds.
There are many variables, and energy is not the only factor to penetration.
Once again, I can point to rifle rounds in water. Much higher energy levels, little penetration.
Heavy hard cast bullets are popular for handgun hunting in tough skinned animals.
Pure science must be backed up with experimentation... and in bullet testing, heavy bullets win.
Thinking on the Spheres of equal size but different weight tests mentioned...
I would bet that drag forces come into play more than with bullets.
The test with bullets into petroleum jelly... Bullets do not fall point first. They fall sideways or maybe base first.
Also at slow speeds the drag forces will be more of a factor, and the heavier is likely to have more, due to its larger size due to added length.
Cavitation effects will play a role in the high speeds of a bullet into a vicious semi solid as well.
A round with higher velocity will cause greater cavitation, resulting from a greater transfer, meaning loss, of energy.
The higher energy and velocity are working against the bullet in regards to penetration.
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If the truck has 1 Joule of KE, you can stop it with an opposing force of one Newton after one meter of movement. 10 Newtons of opposing force will stop it in 10 cm.
There's actually a better way to put it.
A gun and the bullet it launches have equal but opposite momentums after firing. Which would you prefer to stop? The light bullet with lots of KE but the same momentum as the gun, or the heavy gun with little KE but the same momentum as the bullet.
Actually, this is an over simplification and assumes massless gunpowder, in the real world, the gun actually has more momentum than the bullet because it has both the momentum of the bullet and the gasses that drive the bullet, but that only reinforces my case.
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