MadSci Network: Physics

Re: How much force is there behind a bullet?

Date: Fri Jul 13 10:27:03 2001
Posted By: Nauzad Tantra, Undergraduate, Production/ Industrial engg., D J Sanghvi college of engg.
Area of science: Physics
ID: 994867596.Ph

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 

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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