energy vs. momentum

S

simonrichter

New member
While I'm well aware of the difference between energy and momentum in terms of Newtonian physics, I guess I still don't fully comprehend what a practical difference (if any) it makes as far as terminal ballistics are concerned.

E.g. a 9mm and a .45 load may have the same energy, but the momentum of the .45 is much higher because it draws more from the bullet weight rather than from velocity. But does that have any real life effect?
 
Keeping in mind that my math is limited to college algebra/ basic trig, and I have no physics background, my understanding is they are two different measurements.

Momentum relates more to the preservation of energy. As I understand it, if two bullets have the same energy and one has a higher momentum, the one with the higher momentum would be more resistant to being acted on by outside forces. In theory, this would mean that after the same flight time, the bullet with higher momentum would have the same energy.

Ballistic coefficients and other factors that determine the forces acting on that bullet also enter into the equation in a way I'm not knowledgeable enough to quantify
 
momentum can't change form, energy can go from potential to pressure to kinetic to heat and so forth.

Since momentum must be conserved momentum relates much closer to permanent wound cavity(the tissue disrupted directly in the path of the bullet. If two rounds have the same momentum but one has more energy added energy will lead to a larger temporary wound channel as it changes from kinetic energy to elastic and a little bit of heat energy.

As an aside effect from temporary wound channel is unlikely under 500 psi of pressure causing the stretch cavity, possible from 500 psi to 100 psi and probable over 1000.

9mm won't hit 500 psi and deliver adequate penatration.
400px-TBIpwave.jpg
 
Momentum is a pretty accurate way to predict how well a bullet will knock over bowling pins or steel plates. It is darn near useless in predicting how well a bullet will perform after striking a living animal or person.

When it comes to predicting how well a bullet performs inside an animal or person it gets really complicated. Bullet construction and placement are key. You need penetration in order to break body parts. Bullet expansion, or diameter also plays a key role.

If you are comparing very similar bullets then energy can help to predict how much damage a particular bullet will do inside a living creature. It is useless if comparing very different bullet types.
 
so is it save to say that when say a 5,7 and a .45 bullet have the same energy, the .45 is going to be less prone to be set off its flight path when hitting leaves, grass etc.?
 
If you look at historical methods of measuring/predicting "stopping power" - Hatcher's Relative Stopping Power, Taylor's TKO - bullet mass was valued over energy.
It was probably due to the limited types of bullets available in the now-distant past.
I still prefer bullets that are heavy for their caliber, as I trust more in penetration than in "energy transfer" or expansion.
 
Those more knowledgeable than I can correct me here, but my understanding is that neither energy nor momentum are great predictors of effectiveness of a round on living tissue. Rather, the primary consideration is penetration and the size of a hole the bullet can cut (either through original diameter or expansion).
Living tissue is a lot more flexible and adaptive than ballistic gel and therefore does not suffer the same damage due to 'stretch cavity' that ballistic gel does. In other words, as I understand it, there is no such thing as a 'stretch cavity' or hydrostatic shock at the velocities of most handgun rounds (5,7 being a possible exception). My understanding is that hydrostatic shock does not really come into play until 2000 fps or so.
So when it comes to a handgun round effectiveness, you are mostly looking for the hole it cuts (i.e. wadcutters cut better holes than round nose) the diameter of the round through either size or expansion (i.e. some 9mm bullets can expand to the same diameter as a round nose .45) and the distance it will penetrate (preferably 15 inches in living tissue). Heavier bullets smash through bone better (this has to do with your momentum versus energy issue) and bone can mess with the intended expansion of an expanding bullet, so bigger heavier bullets are better in that sense.
It is my understanding that balancing all of these considerations has led the FBI to switch back to 9mm. Better bullet engineering has resulted in more reliable expansion and better penetration than in the past, plus you get more rounds in the magazine than other options.
In other words, momentum is more a measure of inertia, which does not help you determine a bullets performance until you also factor in diameter and velocity.
 
In simplest terms:

Momentum relates to penetration and/or physical knockdown
(as opposed to any CNS or organic response)

Energy relates more to surrounding damage done during that
penetration (which in some cases may actually hinder further penetration)

There's a lot of screaming & yelling about the predictive effectiveness of either one in
isolation. And to be honest, bullet design is the critical factor of how the two play
out against each other as the bullet holds together to penetrate (or not) and/or the bullet
expands to transfer its energy/do damage (or not).
 
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