SpudFiles

Is it the momentum or the kenetic energy that penetrates?

I know its the momentum thats the recoil and the blasting back of the victum but what is the penetration decided by?

For example if a bullet was 0.01kg and traveling at 1000m/s it would have the KE of 5000J and momentum of 10kg m/s.

So would a car with a metal pole the same area of a bullet sticking out the front of it panetrate the same as the bullet if it drove into something at (its mass is 1000kg):
3.16m/s as it would have the same KE
or
0.01m/s as it would have the same momentum.

I was thinking it may be the KE, and even further the J/cm^2.
However this could even be true for kg-m/s-/cm^2 (but thats pretty long).

Thanks for answering.

Hey Jim

Found something for you: http://www.batesville.k12.in.us/physics ... entum.html

Thanks for that Hotwired it was intressting and did state how the kinetic enrgy is the dangerous thing, but didnt mention penetration.

Cheers.

Penetration is a function of many things. If there's one area of research where the search for better penetration is most intense, it's in the field of anti-tank weapons, namely those that function mechanically as opposed to relying on explosives.

Let's take a state of the art projectile as an example, here's a 125mm APFSDS round:



In order to achieve optimal penetration, the following parameters are important:

1) high velocity - if a projectile travels slowly, the target will have time to deform and absorb the blow without penetration, and therefore in order to penetrate well you need as high a velocity as possible. A typical anti-tank round is travelling at around 5,000 fps.

2) high sectional density - in order to maintain its momentum, and transfer the energy in a concentrated manner, you need a projectile that is as heavy and narrow as possible - this is why APFSDS rounds are made of very dense materials such as tungsten or depleted uranium, which being long and thin to give the distinctive arrow shape.

3) high projectile hardness - nature will always choose the path of least resistance, so if the target material is tougher than the projectile, the impact energy will go towards shattering or deforming the projectile as opposed to the target.

edit: fixed illustration

edit 2: again...

Just a quick physics lesson.

Kinetic energy does play a major role in penetration, however the main penetration power comes directly from the surface area of the projectile.

Eg If you shoot a sharpened bullet (low surface energy to Kinetic energy ratio), it will penetrate a lot more than a flat potato (higher surface energy to Kinetic energy ratio)

Hope that helps

A simple comparison is to look at typical airguns in the UK, which are legally limited to the same kintetic energy of 12 ft/lbs.

A typical 0.22 pellet weighing 16 grains travels at 581 fps for 12 ft/lbs.

A typical 0.177 pellet weighing 8 grains travels at 822 fps for 12 ft/lbs.

The 0.177 pellet only has 70% of the momentum of the 0.22 pellet, but will penetrate further by virtue of its higher velocity and smaller diameter.

Ok thanks guys,

So can we conclude that the main factor in penetration is J/cm^?

Pretty much.

1000J on a 0.1cm3 area is a better penetrator than 1000J on 1.0cm3 after all.

More force over a smaller area has the most influence on penetration.

But also hardness plays a part in penetration. A pure lead slug might dent a steel plate, but a lead slug with a tool steel tip mill have more penetration. This is a more important in more powerful weapons.

This can be seen in tungsten tipped bullets and steel cored bullets(a steel rod inside a normal bullet).

LucyInTheSky wrote:So can we conclude that the main factor in penetration is J/cm^?

It's a sum of all the factors I mentioned above case in point:

1000J on a 0.1cm2 area is a better penetrator than 1000J on 1.0cm2 after all.

Not necessarily;

1000J on a 1.0cm² area delivered suddenly by a hard projectile will penetrate more than 1000J on a 0.1cm² delivered slowly by a soft projectile.

In equal scenarios which I expected it to be taken as then my point is true.

I'm aware a good penetrating projectile is hard, dense, thin, long, fast, and stabilised but that had already been posted.

With all other factors being equal, then your statement is correct, but given the fact that there are so many other variables involved that can equally affect penetration in a drastic manner, it's a bit of a sweeping statement to say that J/cm² is the major factor involved.

yeah, this is a good topic, it's something I've been wondering about. If you look at it from an energy perspective, you have KE=1/2*m*v^2. Then you also have E=f*d. So the higher your energy is, and the less force your projectile has to produce to penetrate (ie, make it sharp and keep the diameter down) the deeper it penetrates. So a high sectional density is defintely one of the keys. Which is what makes the arrow such a neat projectile: the old cultures were really onto something! Achieving aerodynamic stabilization and high sectional density all in one.

Heh, yeah, soft projectiles don't penetrate as well, for sure. A potato slug from my N-3 will go through 4 aluminum drink cans, but a marble takes out 10 and keeps right on plowing. (The potato really tore those 4 cans up, though.)

I hate to tell you this, but area is measured in square centimetres, not cubic centimetres.

I'm of the opinion that the factor involved in damage is really pressure on the target. (Not air pressure of course)
This can be figured by dividing momentum by impact time and cross-sectional area.

Essentially this is in proportion to J/cm^2, with a constant defined by target and projectile hardness.

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