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Old 01-16-2006, 10:08   #144
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Originally posted by searcher
Actually, the theory is right. It just doesn't lead to a "knockdown effect" in this case because the "collision" is between a bullet and a soft target. It is a little more apparent in the sport of silhouette shooting where large steel plate animals are knocked over by bullets. In fact, Newton's famous 2nd law commonly stated as "Net Force = mass x acceleration" was originally stated by Newton as "Net Force = (mass x change in velocity) / time" in the special case where the mass is a constant, as in a moving bullet. The term in parentheses is called "impulse" in physics. In words it says that "resultant force equals the time rate of change of momentum." As the time of the interaction approaches zero the force approaches infinity. In other words, the faster the momentum changes, the greater the force.

Another example is the difference in the "kick" felt when you shoot a +p+ round versus a standard pressure round of the same bullet weight out of the same gun. The bullet in the +p+ round gets to any given velocity in less time than the standard velocity round while travelling down the barrel . Same change in momentum in less time yields more felt "kick" or force. Newton's 3rd law about equal and opposite reactions doesn't really explain anything about what happens when the bullet hits the target. The fact that the gun doesn't knock you over doesn't mean diddly. What happens when the bullet is in the barrel and when it hits the target are completely separate events. His second law tells the story, especially in the form he originally used. Sorry to get so technical. The point of my post was to say that even though "knock-down force" from a handgun used for self defense is theoretically possible it can't happen in practice. A reference for the theory is Engineering Mechanics Volume 2: Dynamics fourth edition by J.L. Meriam pages 191-192.
Think of the "knock-down" effect as the work done on the target by the bullet instead of the instantaneous force (delta function, for which the integral over dt is zero)applied to the target.

P.S. Mythbusters is cool
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