Re: Firearm Muzzle Energy
Maybe this will help as well... I'm a college instructor and I teach physics and this question comes up all the time...
Newton's Third Law states, 'for every action there is an equal and opposite reaction.' Taking this into account, if the energy of a bullet were enough to knock over a person then firing the gun would also knock over the shooter.
Coupled with this, of course, is the mass of bullet compared to the mass of the target. Inertia, as understood by Newton's 2nd law is the tendency for an object that is stationary to stay stationary and one that is motion to continue in motion. Now, when two bodies collide their mass is taken into account. I think it's safe to say that a bullet is much less massive than a human body, hence no flying 10 feet backwards.
Finally, there is kinectic and potential energy. While the bullet is travelling it has kinetic energy. When it hits the body that kinetic energy is at its highest amount. If a bullet goes through and through, very little damage is done because the time of impulse (the amount of time the bullet is in contact with the body) is very short. This is why they make bullets that flatten upon impact. This causes the impulse time to go exponentially and also slows the bullet down so that it will bounce off bones rather than go through them. The longer the bullet is moving through the body, the more damage it does. Gruesome, yes, but effective.
I hope this helps.