physics question about bullets

[Woodslab]

Lift them both to a 10 ft elevation and the 4 lb spear will have 40 ft lb of potential energy and the 8 lb spear will have 80 ft lb of potential energy.
As they fall down, the potential energy is converted into kinetic energy. At elevation zero, the potential energy is zero and the 4 lb spear is traveling at a velocity that gives it 40 ft-lb of kinetic energy and the 8 lb spear is traveling at a velocity that gives it 80 ft-lb of kinetic energy.
To give the 4 lb spear 80 ft-lb of potential energy, you have to raise it 20 ft in the air.

When both spears have the same energy which one will penetrate the deepest? The light or heavier or will they penetrate the same?

 

[44 AMP]

I wonder why my 357 mag 125 fmj will go through 3/4" bullet resistant glass and the 180 grn bullet doesn't.

Please explain

Because the glass is a hard target.

And I would guess that the 125 has just enough speed, and the 180 doesn't.

penetration in a hard target (glass, metal, armor plate, etc.,) is different than it is in soft targets (tissue, sand, gelatin,etc.)

The heavy bullet, slower speed more penetration better stopping power only works (and only partially even then) for soft targets.

For some target materials speed is the key factor. For others its not.

Shoot your .357 and a hunting bow at a 3/4" piece of plywood and at a sand bag. The .357 blows through the board like its not there, and stops a few inches deep in the sandbag. The arrow stops dead in the board, and zips through the sand bag like its not there.

 

[Woodslab]

For some target materials speed is the key factor. For others its not.

Shoot your .357 and a hunting bow at a 3/4" piece of plywood and at a sand bag. The .357 blows through the board like its not there, and stops a few inches deep in the sandbag. The arrow stops dead in the board, and zips through the sand bag like its not there.

We are talking about same material same bullet different weights.
If you want to talk arrows than let's compare arrows same size different weight and same energy. Same energy means you have to use two bows.

 

[marine6680]

Hard targets... and bullet resistant glass especially... they dissipate energy differently.


With a relatively thin hard object, speed is more important to getting through it.


As I said before, the mythbusters got a ping pong ball to go clean through a sheet of plywood. Too slow and it would crush and not do much, a little faster and it may dent/crack the wood, but at the right speed, the ball zipped clean through.


Same thing with the bullets. Heavier weight yes, but speed is important. When it comes to hard objects, the differences in weight of same caliber bullets are too small to matter.

 

[Brian Pfleuger]

I wonder why my 357 mag 125 fmj will go through 3/4" bullet resistant glass and the 180 grn bullet doesn't.

Please explain

The easy answer is, that question posits a false premise.

I'd like to see the evidence that a .357mag will penetrate glass that a rifle will not.

These guys would seem to disagree. Note that .357mag is Level 2 protection, where the lowest power rifle listed (.223/5.56) is Level 4 protection.

 

[B.L.E.]

We are talking about same material same bullet different weights.
If you want to talk arrows than let's compare arrows same size different weight and same energy. Same energy means you have to use two bows.

I'm not sure you would have to use different bows. The amount of energy the bow delivers to the arrow is the amount of energy the archer puts into the bow when he draws it. A bow will shoot a light arrow faster than a heavy arrow.
Also, if you claim that a comparison with arrows and bullets doesn't count, you are moving the goalposts in order to make your argument correct. Also known as the "No True Scotsman" logic fallacy.

 

[mehavey]

Thinking about it, the drawn-bow/heavy/light arrows is a good one.
Upon release, the bow will have dumped the same amount of Energy
[½mV²] into each of the two different arrows.

If the 8-oz arrow flies out at 90mph, the 2-oz arrow flies at 180.

'MV' being what it is, however, the heavier carries twice the
momentum as it strikes the sandbag, to penetrate (in a lossless
world) twice as far.

 

[Woodslab]

The easy answer is, that question posits a false premise.

I'd like to see the evidence that a .357mag will penetrate glass that a rifle will not.

These guys would seem to disagree. Note that .357mag is Level 2 protection, where the lowest power rifle listed (.223/5.56) is Level 4 protection

This thread is all over the place next thing will be talking about is fishing boat props.

The polycarbonate(glass) I don't remember the manufacturer but I do remember it said bullet resistent(not proof). It was laminated and 3/4" thick. So all of us welders passed a piece of it around and shot it with different calibers 380,9mm,357,45acp,45 long,44,and 375jdj. Only 2 bullets went through it. 357 fmj(6"rev.) and the 375 jdj contender.

Since you brought up 223
Now, I do chrono my loads and I'm now talking about 357 in carbine. Fastest load 125 fmj I chronod is 2250 fps which if you do a bullet energy calc comes up to 1405 ft/lbs energy @ the muzzle. The 223 remington kicks out 1296ft/lbs muzzle. My rev. 125 fmj 1590 fps and 702 ft/lbs

I don't know about different levels of glass. The glass we shot is probably a lower level.

 

[mehavey]

And poly-carbonate/layered armor (which is what we're talking about here) fails through an
entirely different mechanism than simple penetration through softer homogenous materials.

 

[zombietactics]

If anyone wants to gain actual understanding here, just roll up Woodslab's posts and study them until you understand them.

 

[Brian Pfleuger]

The polycarbonate(glass) I don't remember the manufacturer but I do remember it said bullet resistent(not proof). It was laminated and 3/4" thick. So all of us welders passed a piece of it around and shot it with different calibers 380,9mm,357,45acp,45 long,44,and 375jdj. Only 2 bullets went through it. 357 fmj(6"rev.) and the 375 jdj contender.

Hard to say for sure. One piece of glass being shot all those times is a bit inconclusive. Besides which, depending on how the 45colt was loaded, there's a very good chance that the two highest momentum rounds went through the glass.

In any case, a very small sample size with unknown materials.

Now, I do chrono my loads and I'm now talking about 357 in carbine. Fastest load 125 fmj I chronod is 2250 fps which if you do a bullet energy calc comes up to 1405 ft/lbs energy @ the muzzle. The 223 remington kicks out 1296ft/lbs muzzle. My rev. 125 fmj 1590 fps and 702 ft/lbs

Note that the momentum of the .357mag is considerably higher (40 vs 25) but the sectional density of the .223 is higher (I'm assuming a 55gr bullet) at .158 vs .140.

 

[Woodslab]

To simplify this a bit instead of using JHP bullets, let's use regular ball ammo. Exact same bullet, exact same muzzle energy (500 lbs let's say) just different weight (155 and 180 gr) and different speed. Which bullet will travel farther through ballistic gel.

Getting back to the question.
Look at it this way @ the muzzle these 2 different weighted bullets have 551/556 ft/bls. energy. the 155 bullet is tweeked up to 1271fps to get the 556 ft/lbs.

Now we have and again no exspansion
155 fmj 1271 fps 556 ft lbs
180 fmj 1180 fps 556 ft lbs

So both fired,one is going 91 fps faster. That speed makes up for the loss of weight. The slower bullet has its energy stored in weight. The 155 hits the gel fast and slows quickly. The 180 hits hard and slow. End result same depth

556ft/lbs is 556ft/lbs however you look at it. I don't know where you guys get the extra penetration energy.

 

[Ruger480]

It comes from the extra momentum carried by the heavier bullet.

 

[shortwave]

All things being equal with the exception of bullet weight, my money for deeper penetration would be on the heavier bullet.

 

[zombietactics]

... Now we have and again no exspansion
155 fmj 1271 fps 556 ft lbs
180 fmj 1180 fps 556 ft lbs

So both fired,one is going 91 fps faster. That speed makes up for the loss of weight. The slower bullet has its energy stored in weight. The 155 hits the gel fast and slows quickly. The 180 hits hard and slow. End result same depth

556ft/lbs is 556ft/lbs however you look at it. I don't know where you guys get the extra penetration energy. ...

This is unarguable logic from physics.

The misunderstanding of some possibly comes from a lack regarding realization of the proportionality of mass and velocity for a given KE.

Another way to look at it is that speed "makes up" for a lack of momentum, and momentum "makes up" for a lack of speed.

 

[Brian Pfleuger]

This is unarguable logic from physics.

The misunderstanding of some possibly comes from a lack regarding realization of the proportionality of mass and velocity for a given KE.

Another way to look at it is that speed "makes up" for a lack of momentum, and momentum "makes up" for a lack of speed.

The misunderstanding is that folks think kinetic energy has some bearing on penetration. That is a false notion.

Momentum and sectional density are responsible for penetration.

Speed does not "make up" for a lack of momentum and momentum does not make up for a lack of speed.

Kinetic energy is NOT responsible for penetration.

 

[Woodslab]

Did my posts penetrate or bounce off?

 

[marine6680]

This is unarguable logic from physics.

The misunderstanding of some possibly comes from a lack regarding realization of the proportionality of mass and velocity for a given KE.

Another way to look at it is that speed "makes up" for a lack of momentum, and momentum "makes up" for a lack of speed.

Go back and read my post here.


A force opposing the vector of momentum is what stops an object.

Force is the counter to momentum... not energy, nor vice versa.

penetration is governed by deceleration... Deceleration is a force... force is a quantity that is applied over time. F= mass times acceleration/deceleration

Applied over time...

An object that resists such force will travel farther before stopping.

 

[Brian Pfleuger]

Also note that force can be expressed in terms of momentum..

f=∆p/∆t

force equals change in momentum divided by time.

Which also means the ∆p=ft

The change in momentum equals the force applied times the time the force is applied.... "deceleration" in this case, in common terms.

(Technically, any change in speed is an acceleration in physics terms. There is no "de"celeration.)

 

[zombietactics]

Momentum and sectional density are responsible for penetration.

Easily disproven ... it's a classic physics problem:

What is the more dangerous to stand in the way of:

• A 20-ton truck coming at @ 0.1 m/s? (Momentum= 2000, KE= 1 Joules)
  
• A 20 kg 1 sq/m metal plate coming at you @ 100 m/s ? ( Momentum= 2000, KE= 1,000,000 Joules)

Provided the truck is not accelerating under power, or coming down a hill ... you can probably stop it with some effort. You need to overcome momentum, which is conserved/transferred by your pushing back on it.

There isn't much you can do about that steel plate ... you're getting squished.

The momentum is identical in both cases. The surface area which contacts you in both cases is identical. The difference is KE.

If hardness & density are equal, and a higher penetration is desired, then Kinetic Energy spread over as small a surface area as possible is what is required.

This holds 100% true experimentally, as well as according to the laws of physics.

With no disrespect for anyone here, this is settled science. There isn't even a coherent hypothesis for how it works otherwise.