I was playing around in Wikipedia and found something attributed to Sir Isaac Newton that I don't recall having heard of before. (Which is understandable since I did sometimes doze in physics class. )

I don't know how I really feel about it, seeing how I just have found it, but here it is.

The gist: Newton's estimator says that the depth of impact (penetration?) is proportional to the projectile length x specific density* x 4, and leaves out everything else. (????)
(*specific density is the density of the projectile divided by the density of the target)


Now if it had come from anyone else, I would have had to raise a flag:



But coming from Newton, who pretty much was physics, I have to wonder...

To my mind, it would leave out way too many variables, not the least of which is the tensile strength of the target.

But, anyway, whatever it is, I thought it would be good fodder for discussion.

P.S. This is about guns, not flamethrowers...

As an approximation it seem accurate enough. At least according to how I experience effects of shooting into different media. Whats said about cohesion could probably be transferred to tensile strenght. Its the velocity that does it all. When the velocity is high enough all transferred energy is used to deform the media. If you shoot at a metalplate about 5 mm with a 22 lr it falls over, if you shoot it with a 30.06 it wont move at all, just get holed.

Now over to the guys who know what they are talking about.

quote:

Now over to the guys who know what they are talking about.

Oh, that wouldn't be me, I assure you.




Now that I've had a chance to think about it, here are two issues brought out in the article:

quote:

This approach only holds for a blunt impactor (no aerodynamical shape)

and

quote:

It is then important that the projectile stays in a compact shape during impact (no spreading).

At first glance, these facts would pretty much seem to eliminate most bullets right away, unless our projectile was a high-velocity can of pork and beans. However there is often some blunting of a projectile after impact, mainly with hollowpoints, softpoints, etc. So this equation still may have some validity, at least some of the time.

Or so I think.
(See my sig line below for Moe Howard's caveat about some people thinking.)

quote:

I thought it would be good fodder for discussion.

...or not.

But forging right along:

I got some interesting results from looking at this page, from a site incidentally belonging to one of our own esteemed members. The lengths of those particular bullets are laid out very conveniently.

If you're interested, here's a hint: Use the value 8.9 for the specific density, which is pretty much the density ratio of copper to soft tissue.

Or not...

'Nuff said.

I'm still interested.

I have often been philosophying about what happens right before the bullet stops. My idea is that the bullets stops fairly soon after the velocity falls below a certain threshold. Suddenly the velocity is to low to tear tissue. Often one can see the bullet lying under the skin on the far side with more or less skin punched loose from the body. Is also interesting that when the bullet exits, almost no skin is "loose" around the exithole, suggesting that Newton was on to something when he said that cohesion is of little interest when the velocity is high enough.

Barnesbullets losing petals would be close to the theorys projectile. If only we could get homogenous game.

We need a geek.

quote:

Originally posted by 900 SS:
I'm still interested.

I have often been philosophying about what happens right before the bullet stops. My idea is that the bullets stops fairly soon after the velocity falls below a certain threshold. Suddenly the velocity is to low to tear tissue. Often one can see the bullet lying under the skin on the far side with more or less skin punched loose from the body. Is also interesting that when the bullet exits, almost no skin is "loose" around the exithole, suggesting that Newton was on to something when he said that cohesion is of little interest when the velocity is high enough.

Barnesbullets losing petals would be close to the theorys projectile. If only we could get homogenous game.

We need a geek.

Geek? No sir, I've retired.



Thanks for your interest, 900.

I think the low threshold for the penetration velocity is when the deceleration equals that of gravity. On its course through the target, the bullet is decelerating dependent on the velocity, among other things, and when the deceleration equals roughly 9.8 m/s^2 is the point where we can assume that the bullet stops. (This is similar to the notion of terminal velocity.)

Whether that is right or not, it certainly does help when you are trying to come up with a mathematical model using a computer program. When you try to solve for penetration depth you quickly find that your velocity never equals to zero. Very inconvenient.

It's the velocity threshold that's also puzzling about Newton's equation. There must be a point where the penetration drops off when the velocity is down to a certain level. It would be nice to know what that level is.

Some of my best naps in university have been interrupted by physics lectures.

So with that caveat...

From what I have been able to gather Newton's equations are basically mathematical expressions of his laws. While these equations are "valuable" for their ability to explain physical laws they ignore such things such as friction and thus to not give real world answers.

Perhaps a realistic formula could be developed as a basis but I guarantee it would be very long and very painful.

Did you know that anyone can post, edit or change any subject on Wikipedia? It's the last place I'd look for an absolute answer.

/

quote:

Originally posted by whizzbang:
Some of my best naps in university have been interrupted by physics lectures.

So with that caveat...

From what I have been able to gather Newton's equations are basically mathematical expressions of his laws. While these equations are "valuable" for their ability to explain physical laws they ignore such things such as friction and thus to not give real world answers.

Perhaps a realistic formula could be developed as a basis but I guarantee it would be very long and very painful.

You are correct a realistic formular must take into account any and all variables nd therefore is rather complicated. In real collisons kinetic energy is calculated in Joules (joules is the international unit for thermal energy[heat]) witch is the form that most of a projecticles kinetic energy is tranfered into in a real collision..

penetration is dependent on momentum. Momentum is is the combination of velocity and mass. (mass might be the specific density part) You gain more momentum, and there for penetration, from the mass part. There is resistance force that eats away at your velocity. The faster something goes the faster is slows down when it hits resistance. Stick you hand out the window going 60 and then 30, where do you feel the most resistance?

I don't know if this is what we were looking for.

i thought newtons idea here
had to do with force applied to the tip
of an object at contact.?