# Re: PHYSICS: Getting knocked on your ass (was Re: TERRORISM: looking forsolutions)

From: Doug Jones (djones@xcor.com)
Date: Fri Sep 14 2001 - 15:53:07 MDT

James Rogers wrote:
>
> Part of the problem is that you are confusing energy with power. A punch
> doesn't have a lot of power but it does have lots of energy. A pistol
> bullet has a moderate amount of power, but minimal energy. Energy is what
> knocks you on your ass, power is how its delivered.

I'm afraid you're mistaken, James. The terms you need are energy
(0.5*mv^2) and momentum (mv). A fist carries quite a bit of momentum
due to its mass, but little energy due to the low velocity. Bullets are
low mass, high velocity. For example (rough guesstimates):

mass velocity momentum energy
Fist 1 kg 10 m/s 10 kg-m/s 50 J
Bullet .003 kg 500 m/s 1.5 kg-m/s 375 J

Thus a blow with a fist or foot is likely to impart substantial momentum
to the target, while a bullet penetrates and tears it while not causing
much movement.

```--
Doug Jones, Rocket Plumber
XCOR Aerospace
```

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