Momentum vs Energy

Discussion in 'The Powder Keg' started by Ron AKA, May 19, 2008.

  1. Ron AKA

    Ron AKA Guest

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    I don't want to stir up another big bore vs small bore, or heavy bullet vs light debate, and only intend this to perhaps shed some light on what momentum is, what energy is, and how they differ.

    I found a couple of YouTube videos done by a "mad professor" type guy, that are quite informative, if you watch both of them through. He uses a couple of good examples; one with two cars attached by a spring, and another of shooting a gun.

    Here they are if you can spare the time to watch. Each is about 7 minutes long. The first one takes about 30 seconds to get going.

    [ame="http://www.youtube.com/watch?v=VKCdq6X08Sw"]YouTube - Julius Sumner Miller - Physics - Energy and Momentum pt. 1[/ame]

    [ame="http://www.youtube.com/watch?v=5Yl4SskTl9E&feature=related"]YouTube - Julius Sumner Miller - Physics - Energy and Momentum pt. 2[/ame]

    Some comments on the two examples. When he stretches the spring connecting the small light car to the big heavy car, and lets it go, the two cars are pulled together by the same force. The heavy car goes slower, and the light car faster, but they have the same momentum (mass times velocity). However, the key major difference between the two is that the force acts on the small lighter car for a much longer distance. That gives the small car much more stored energy that is real and available to do work. He goes on to explain how work is done and where the energy goes.

    In the second example of firing the gun which is in the second video, which is probably of more interest here, the same thing happens as the light car and heavy car connected by a spring. The spring in this case is the expanding gases in the barrel of the gun and of course the gun/shooter and the bullet are going further apart insead of coming together (a good thing). At the point the bullet leaves the barrel the much heavier gun and shooter are moving back a very short distance at relatively low speed, while the bullet is going very fast. The momentum is equal however, just like in the case of the two cars. The energy is not, as the force has acted on the bullet for a much longer distance, so it has much more stored kinetic energy.

    This is important to consider. You and the gun have just taken the full momentum of a bullet at muzzle velocity. And on a big gun while not real pleasant it does not kill you. If you were to stand in front of the bullet, things would not be so pleasant, and it most certainly would kill you. The reason is that the energy in the bullet is huge compared to the energy you absorb at the other end of the gun.

    So in short the gun/shooter have the same momentum as the bullet. The bullet however has a huge amount more energy due to the distance the expanding powder has acted on it.

    I hope that makes a bit of sense??

    Ron
     
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  2. TXplt

    TXplt Gun Toting Boeing Driver Forum Contributor

    Thanks, Ron. I found that informative and interesting.

    But if you were to take the full amount of energy the bullet has over a large surface area it stands a good chance of being uncomfortable, but not killing you (hence body armor). It's the unique ability to punch a hole and transfer momentum, shock and energy over a path that makes a bullet lethal (and also determines its stopping power). This is why shot placement is critical, and no one has really succeeded in figuring how good a stopper a bullet is, except through empirical studies.

    The other thing to consider, at least as far as recoil goes, is how much momentum is delivered to you the shooter over time. This determines if, given equal momentum and gun weight, the shot (to the shooter) is a "push" or a "smack." This (as well as noise and blast) is why recoil is tough to quantify as well.

    :)
     

  3. Yep some good info! Thanks for posting!
     
  4. samuel

    samuel G&G Newbie

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    I guess i am just stupid but how can anything be the same velocity or have equal energy when one is moving faster than a speeding bullet and the other is at a very low speed?And furthermore if the gun were moving the same velocity as the bullet as this states,I'll bet it would tear your shoulder off.And how does the propelant act on the bullet any longer?I bet if the gun were lighter than the bullet,the gun would be traveling fast and the bullet would be going slow.This proves to me that the expanding gasses are acting equally on both.In other words,both are under pressure for the same period. sam.
     
  5. TXplt

    TXplt Gun Toting Boeing Driver Forum Contributor

    sam--

    momentum=mass x velocity

    Kinetic Energy = 1/2 x mass x (velocity) squared

    So, it's possible for a fast bullet to have the same energy as a car going slow.

    KE changes a bit as you get near the speed of light, but we don't need to worry about that for most terrestrial stuff because it has to be very close to light speed for the relativity stuff to kick in (this is why an atomic particle, theoretically as it got very close to the speed of light, while tiny in mass could have as much energy as a speeding locomotive).
     
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  6. Ron AKA

    Ron AKA Guest

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    Yes the powder expansion pressure acts on the gun for the same length of time as the bullet. This is the same in heavy car, spring, light car example. The time the force acts is exactly the same. The difference is the distance. Because the same force acts over a longer distance more work is done on the light car or the bullet, which means it has more stored kinetic energy. That is the key to the whole thing.

    Not wanting to get the discussion side tracked, but there is a similar situation in combustion engines with the torque vs horsepower discussions. Take two engines with the same torque at 6000 rpm. This works out to a specific horsepower at that rpm. Now if you double the speed of one engine without changing the torque to 12000 rpm, you will get much more horsepower. The time is the same, but the distance the force (torque) acts over is twice as far.

    Ron
     
  7. AKHunter

    AKHunter G&G Newbie

    The Prof. was entertaining to watch, he sure enjoys physics. I was concerned at first, it took a few minutes to enter friction into the explaination.

    The thread title took me aback a bit "Momentum vs. Energy" since momentum cannot exist without energy. Not wanting to nitpick at all but M AND E makes it clearer.

    Thanks for sharing, fun to watch the demonstrations. Particularly the failed energy ball experiment. :)
     
  8. samuel

    samuel G&G Newbie

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    Ah,but it is easier to worry about the bullet action before it happens and the recoil after it happens.Sometimes(most times)deep penetration is less likely to kill dead than less penetration,good expansion,and wider wound channel.I have never had the priveledge to hunt either cape or asian buffalo,but I have often wondered if the fact that the hunters(especially on cape buffalo)were using solids for max in penetration instead of frangible bullets like we use on our larger game in North America.We have much larger primary wound channels,even if they don't go all the way through. sam.
     
  9. Ron AKA

    Ron AKA Guest

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    Since posting this a while back, I came across this interesting device:

    Armour-Piercing Fin-Stabilized Discarding Sabot

    Kinetic energy penetrator - Wikipedia, the free encyclopedia

    It is an excellent example of how bullet design converts kinetic energy into a very destructive force -- enough to penetrate tank armor. They have a velocity of nearly 6000 fps, which is the main contributor to their high energy.

    While very differerent that a hunting load, it does illustrate the power of kinetic energy and how a bullet needs to be carefully designed to take advantage of it.

    Ron
     
  10. JMcDonald

    JMcDonald Suspended

    I had a teacher in one of my classes that acted and talked and even looked almost exactly like that. He even used much of the same vocabulary, haha. He was a Strength of Materials professor.

    Interesting videos and discussion!
     
  11. Bravo

    Bravo G&G Newbie

    While we're on physics topics, we can take Newton's 3rd Law and create an equation to tell us how much energy a rifle or handgun will recoil with a given bullet/powder charge.

    M1*V1 = M2*V2

    M1 = Mass of Rifle
    V1 = Velocity of Rifle moving into your shoulder
    M2 = Mass of Bullet (grains will have to be converted into kg)
    V2 = Velocity of Bullet

    Let's use a rifle with the fewest moving parts and no muzzle break...I think moving parts reduce recoil, although energy may be conserved and still thrown into your shoulder...I don't know for sure.

    In this case, a Remington M700 BDL chambered in 30-06 Springfield.

    Mass of Rifle = 3.35kg
    Velocity of Rifle into Shoulder = ?
    Mass of Bullet = 150grain = 0.0097kg
    Velocity of Bullet = 883.92m/s

    Solving for Rifle Velocity => (883.92m/s * 0.0097kg) / 3.35kg = 2.559m/s = 5.7mph into your shoulder.

    Now we take the solved velocity and create some energy numbers...

    Kinetic Energy = 0.5MV^2
    Kinetic Energy for rifle = 0.5(3.35kg)(2.559m/s^2) = 10.969 Joules.

    This number doesn't mean much, until you calculate what the object on the other end eats up.

    Kinetic Energy of Bullet = 0.5MV^2

    => 0.5(0.0097kg)(883.92m/2^2) = 3789.38 Joules

    In other words, the recoil of a bolt action 30-06 may make your shoulder sore, but being hit by the bullet will make you more sore :D
     
  12. samuel

    samuel G&G Newbie

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    Huh?or pardon me?(Depending on whether you are northern or southern) sam.
     
  13. Bravo

    Bravo G&G Newbie

    In simple terms:

    The butt of the rifle doesn't go through your shoulder like a bullet would because of the rifle's mass (and surface area, but that's another equation).
     
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  14. samuel

    samuel G&G Newbie

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    Then having 20lbs recoil on the butt and 2800ft lbs on the bullet has nothing to do with penetration? sam.
     
    Last edited: Jun 27, 2008
  15. Bravo

    Bravo G&G Newbie

    Penetration of a bullet is determined by its ballistic coefficient (mass, surface area, and drag) and velocity.

    The weight of the rifle has nothing to do with what the bullet will do down range. It has everything to do with how your shoulder will feel after firing the rifle.
     
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  16. Mooseman684

    Mooseman684 G&G Newbie

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    I will dispute the Statement that penetration has to do with its BC and velocity...
    In a General Term... I can speed up a bullet so fast that it will explode on Impact regardless of its BC...
    Ballistic Coefficient is the ratio of the sectional Density of the projectile to its Form Factor.It is an Index of the manner in which a particular projectile decelerates in free flight due to the resistance (Drag) of the atmosphere in which it is traveling...Has nothing to do with Penetration !
    Bullet design , Hardness, and Velocity have to be considered as well as what you want to penetrate...
     
  17. samuel

    samuel G&G Newbie

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    I think you misunderstood me altho I dont know how.I will edit it and make it more clear.I meant 20lbs recoil and 2800ft lbs.Anyway if you reversed it and had 2800lbs recoil and 20ft lbs energy,I bet your shoulder would hurt then,no matter how wide the butt was. sam.
     
  18. Ron AKA

    Ron AKA Guest

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    Sam you are correct. I think we had this discussion earler in the thread. The issue is that the force acts on the bullet for a much longer distance than the distance the force acts on the shoulder. As the "mad professor" explains in the video, this is why there is much more energy in the bullet than there is in the gun acting on the shoulder.
     
  19. Bravo

    Bravo G&G Newbie


    Good point. I probably shouldn't have jumped out there and made that statement.

    I guess I was thinking in simple terms, as in the wider .45 slug doesn't penetrate as deep as a narrower .30cal slug, but I knew there was more too it :D
     
  20. Ron AKA

    Ron AKA Guest

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    Bravo, while not totally right, I think you were very close. Sectional density is probably the best measure of penetration ability. However, ballistic coefficient and sectional density are very close on a relative basis if the bullet shape is the same. BC is improved by the shape of the bullet (boat tail, very low drag shapes), which is not reflected in the sectional density. This said, I think the more important factor is bullet design and how much weight is retained, and how much the bullet expands. Sectional density is measured as the bullet comes out of the box -- not as it passes through the game, so I'm not so convinced as to how important it is.

    In the article on armor piercing bullets, some very interesting design details are explained. One is that the depleted uranium or modified DE actually reduces diameter or has negative expansion as it goes through the armor. This is a good example of how bullet design is critical to use the available energy for the intended purpose.

    Ron