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Posted: 4/20/2006 3:22:22 PM EDT
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I'm not a bullets kind of guy, I know maybe just enough to keep myself able from being called a moron. But the point is, I dont really know. However, I do know when I shoot my AR10 at steel targets, it hits them hard. If its a hanging target, it whacks it very well, if its my standing target, it maybe gets knocked over. Anyway, some guy in AR Discussion posted this linkeroo Guy with body armor supposedly gets shot point blank with a .308. He stands on one foot to show how it wont really do anything. Is this for real? And if so, why will it knock my pepper popper which weighs maybe 70 pounds with the stand and all to heck, and this tall skinny dude doesnt even flinch? I apologize in advance is this is a dumb question. |
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Can the frame brace itself, rebalance itself, or actually have the round make a depression that then fills back out? I can shoot my swinging plate with a .454 and it won't get knocked over. If I welded it solid I'm sure it would flip off it's legs as the impact would not be absorbed and the energy transferred into motion. [plate swinging] |
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Spooge5150 has the right idea. The impulse of a bullet impacting point blank will be the same as the recoil impulse. Impulse is a quantity that represents force applied over time. As gas expands in the barrel, force is applied to the bullet. An equal and opposide force (recoil, if you will) is applied to the rifle, which is in turn applied to the shooter. Obviously, force on the bullet and the force on the rifle are applied for the same amount of time. The impulse applied to each is therefore the same. If a given impulse is applied to a mass, that mass will start to move (provided that it is free to do so). The resultant motion of that mass is quantified with a property called momentum (mass x velocity). Impulse and momentum should not be confsued with work and energy. Work is a quantity that represents force applied over distance. The work done on a bullet results in the kinetic energy of that bullet. While force is applied on the bullet and rifle for the same amount of time, it is not applied over the same distance. The flying bullet therefore has more energy than the rearward motion of the rifle/shooter. Both energy and momentum are conserved. When a bullet strikes a free object (say a dude standing on frictionless rollers), we can predict the rearward momentum of the dude -- it will be the same as the momentum of the bullet just prior to impact. The kinetic energy of the dude traveling rearward, however, will not be the same as the kinetic energy of the bullet just before it impacts him. But energy is conserved, you say! Correct, but a good portion of the initial kinetic energy of the bullet is used to f*&^* the dude up (a lot of work is done on his insides, etc). Anyway, the basic point is that momentum is important for a projectile to move something, while energy is important for a projectile to do damage. The peak force of a bullet impacting most objects will be very, very high, but that does not mean that it will move that object a great deal. The reason here is that those forces act over a short period of time (again, impulse). Try this: hang a steel plate so that it is free to swing. Shoot it and see how much it moves. Now take a brick and throw the brick at the plate. Unless you're a major pussy, the plate will move substantially more after being hit by the brick. The brick has more momentum. I'm sure we'd all choose a brick in the chest over a bullet, though. Why? The bullet has more energy. It will do more damage to the plate. Anyway, I wish I had the time to write more here. An entire book could be dedicated soley to projectile dynamics. ETA: My bullet/brick example is not the best because it does not take fully into account inelastic vs elastic collisions. It should, however, give you the general idea. |
Thats usually soft vests with pistol bullets, much smaller area the pressure is spread over and there is also no back face deformation with a rifle plate. |
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