|MadSci Network: Physics|
Hi Andrew, There is a law in Physics which says that the momentum of a system always remains the same. It could be explained as follows. Suppose a billiard ball is hit towards the other balls. The ball will have some momentum (momentum = mass * velocity). When it strikes the target it will transfer some of its momentum to the target, (the target being the other ball). Accordingly only the target ball, or both of them will move along the table. However when a bullet is shot, things change a little. In the earlier case we notice that both the balls are very hard in nature and one ball cannot penetrate the surface of the other ball. A bullet is made such that it can easily penetrate the skin and slice through it. In such cases the momentum lost by the bullet may be very small (in which case the bullet will come out of the rear of the persons body). However if the bullet hits something impenetrable like a big bone, or if the speed of the bullet is very very large, or maybe it is big in size (big mass) then the chances are that the person will move backwards. In simple terms a person will move backwards (or fly :) ) if the bullet transmits momentum which is high enough to move the person. Lets take an example. Say the bullet is 100 gms in weight and travelling at the speed of 600 m/s (Thats nearly twice the speed of sound). If the person standing in its path weighs about 60 kg. and the bullet stops inside his body, then he would move backwards by a velocity of about 1 m/s. However if there was a kid in the path (20 kg in weight), and the bullet transmitted its full force onto him, then he would move at the speed of 3 m/s (which would make him "fly" back). Hope this answers your doubt. If you need any clarification please feel free to write back to MadSci. Nauzad Tantra
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