Home Forums Hunting & Shooting Elk Hunting Momentum versus Energy

Anybody around that believes momentum rather than calculated energy is the key to knock down power? It explains why some older guns got a reputation for being good stoppers while some newer loads with more (energy) are not as reliable for knock down.

The English were one of the first to advocate this..their story was to take a bank vault door and hit it as hard as you can with your fist and see how far it moves, then to push it with your palm and see how far it moves....some sense to that, it seems to me... Roy Weatherby of course took exception to that of course..

both are relevant to mass and velocity... neither mean squat when it comes to the 'myth' of knock down power.

both are relevant to mass and velocity... neither mean squat when it comes to the 'myth' of knock down power.

If you think "knockdown power" is a myth (and it is), a simple conservation of momentum analysis done on a bullet striking a big game animal will demonstrate quite well that it's a myth. So why do you say it doesn't "mean squat"?

With hunting season over, it means 10 more months of arguing this minutae.

Sorry didn't mean to cause confusion. What I meant by doesn't mean squat, is that neither one are worth the time thinking about. Put the bullet where it needs to go and be done with it.

I 'knock down' about 6-10 critters a year on average (only 5 this year... been kind of slow). I've 'knocked down' animals with a 22-250, and ''knocked em down' with a 375H&H. All fell over dead, and to be honest, I was not impressed with the knock down power of the 375. I'd have much rather had a 7mag and some Accubonds. <img src="/ubbthreads/images/graemlins/grin.gif" alt="" /> At least I can see the animals reaction when it hits them. <img src="/ubbthreads/images/graemlins/smile.gif" alt="" /> I've even knocked down deer with 22mag. (for the internet police, there is no minimum in Montana)

I feel comfortable shooting any critter with a rifle capable of penitrating the rib cage and destroying lungs or penitrating the skull (in the case of the doe in MT I shot with the 22mag at 10 yards). Knock down power doesn't kill chit! If that was the case then a muzzle loader would be useless against animals such as deer, elk or moose.

Personally I think that people see the animals reaction when struck with a lightly constructed bullet and correlate it too knock down power. I've seen some serrious bang flops on elk with 'inadequate' cartidges/bullets, and seen many critters run off after being shot with cannons. A friend of mine shoots a 340 Roy, he shoots 250gr noslers at elk, and I've watched him shoot 4 with that thing, and you'd think he was missing. Every one ran off one ran over 300 yards with a hole in both lungs. A 340 has some serrious knock down power doesn't it????

I shot a kudu last year in RSA with my 375, twice in the ribs, and if I didn't hear the bullet hit the bone I would have thought I missed by the reaction he gave. Why didn't a 300gr bullet knock him down?

Carry on... Its still hunting for the next month and a half for me, and I have 3 more freezers to fill for people. While you guys are disecting the theroy of knock down, I'll be 'knocking down deer with my bow/muzzle loader/ rifle.

Maybe I don't understand what knock down power is? I've killed enough critters to know that you put it where it needs to go and worry about it later.

The Taylor index is what I use to "compare" bigger bore guns that sling big chunks of metal relatively slowly (elk loads):

http://www.loadammo.com/Topics/July02.htm

I'm not going to argue energy versus momentum, as I think they are just two factors in a much more complex relationship. More important, IMHO, is a balance of various factors.

Velocity, energy, momentum, bullet diameter and weight, bullet construction and shape, animal size and build and bullet placement all play a role indetermining terminal results. There are other factors, too, like the animal's general health and arenaline levels, etc.

In the end, however, energy must be transferred to the target or no damage is done - that is basic physics and if you think differently you should use your ideas to develop a perpetual motion machine. And get incredibly rich.

Other factors being equal:
* Energy applied slowly may have dramatically different results than energy applied quickly.
* Transferring more energy is generally going to result in more changes to the target (damage) than less energy applied over the same time period.
* The heavier bullet will penetrate deeper.
* A bullet that expands slowly will penetrate deeper than one that opens quickly.

Being a physicist, I am unaware of your theories of physics. Where does the energy come from? Try momentum. Momentum must be conserved is the first law, not energy. All analysis starts and ends with momentum. How do you lose momentum? By transferring it to energy. Energy does not break an elk shoulder. If you cannot break the shoulder you do not get to the heart and lungs. Momentum does break the shoulder. Look up impact or impulse analysis and you will learn something about killing an animal. There are two forms of collisions: elastic and inelastic. Bullet deformation upon impact is a loss of momentum by transferring energy to deform the bullet (as opposed to breaking the shoulder bone). That is why a small light expanding bullet will not break a large heavy bone as well as a heavy slow solid one.

Actually, they are not my theories.

As a physicist, are you saying the concept of conservation of energy is foreign to you? (The First Law of Thermodynamics comes to mind...) Or is it that you are unfamiliar with the definition of energy as the ability to do work?

So in your world energy deforms the bullet but Newton's Third Law is suspended in regard to bullet impact? Please explain?

By the way, I can easily get to the heart and lungs without breaking a shoulder ...

You just had to do this, didn't you? <img src="/ubbthreads/images/graemlins/frown.gif" alt="" /> How many fairies can dance on the head of a pin? <img src="/ubbthreads/images/graemlins/smirk.gif" alt="" />

First one with the correct answer gets a cookie! <img src="/ubbthreads/images/graemlins/smile.gif" alt="" />

My 6 year old daughter says Tinker Bell, the Sugar Plum fairy, and the Tooth Fairy, will fit on the head of a pin, so the answer is 3. Now she wants her cookie!

Nah, There's at least 4. She forgot the fairy godmother, but I bet she's still smarter than the rest of us though! <img src="/ubbthreads/images/graemlins/grin.gif" alt="" />

Coyote hunter.

Many laws of physics. Need to know which one applies when.




I would prefer a slice of pecan pie rather than a cookie.

OK, now that we are back on the right track, let us understand how to break an elk shoulder using momentum analysis.

It is easier for me to just cut and paste from wikipedia than to retype from a physics text.

Are we all right so far?

Bullet has a given momentum as it is close to the elk. Elk has no momentum. Collision of bullet with elk is inelastic because the shape of the bullet and the elk is altered. Kinetic energy is required to alter the shapes so a part of the momentum is converted to energy to deform the bullet (hunters call it bullet expansion) and some to deform the elk hide and muscles (hunters cal this an entrance wound). One recalculates the momentum remaining for the bullet as it nears the elk shoulder bone. Elk shoulder bone has no momentum. Using an impulse analysis determine if there is enough momentum left to provide the force needed to break the shoulder.

Now then the reader who is awake will scratch their head and ask �Who the heck knows how much force is required to break a shoulder bone?� That is the subject of a different day but one can make a good estimate of that force if one analyses enough elk shot in the shoulder.

OK, back to the subject. Once we know the momentum transferred to the shoulder bone during the breaking, we must recalculate the momentum of the bullet (and or bone fragments which have gained some momentum and are traveling in the general direction of the bullet into the chest cavity). The bullet must have enough momentum to have another impact with ribs which are not too tough and muscles and still keep moving into the chest cavity where tissue is generally soft and will not cause much conversion of momentum to energy.

Finally, there are the people who demand an exit wound. Those people must be prepared to shoot a bullet that is resistant to fragmenting and has enough momentum to not only keep moving after the initial collisions with the shoulder and ribs going in but survive two additional collisions with ribs and shoulder bones coming out. This is not any harder to calculate than the none exit wound model, you just have to double your earlier calculations and add a few losses of mass and energy as the bullet loses weight and increases diameter.

Ahhh.... "Kinetic eneargy is required..."

Kind of what I said to begin with.

If the bullet does not transfer energy to the animal, A) the bullet is not deformed and B) neither is the animal.

Zero energy transfer = zero damage.





[Typo corrected.]

When I started the thread, I just wanted to see if anyone out there was awake.

Two bullets can leave the barrel with the same kinetic energy and very different momentums, and in guns of equal weight the one with more momentum kicks harder. Buffalobob comes closest to having it right and therefore wins the pecan pie made with a liter of cognac. <img src="/ubbthreads/images/graemlins/smile.gif" alt="" />

If my ballistic calculator is correct, the one example I checked verifies your statement about the load with the greater momentum recoiling more. Not that that is very surprising, as I have often felt and stated that momentum is at least as good an predictor of penetration potential as is velocity. (I also believe neither is a perfect predictor.)

For fun, though, let's compare two loads, both using equal amounts of powder (58.5g is what I used):
.300 Win Mag, 168g @ 2589fps, 2501fpe, 1.0 relative momentum (this is very close to my .308 Win equivalent load)
.45-70, 350g @ 1794fps, 2502fpe, 1.4 relative momentum (this is very close to one of my medium loads)

Shoot both of these loads from a 7.5 pound rifle and the recoil is as follows:
.300 Win = 21.29 foot-pounds @ 13.52fps
.45-70 = 34.44 foot-pounds @ 17.20fps

All of which has absolutely NOTHING to do with my original contention that "energy must be transferred to the target or no damage is done".