🚀 go-pugleaf

hhtra@usho21.hou281.chevron.com (T.M.Haddock) writes:

> Over on the REC.GUN newgroup, there is a thread running about Bullet
> Proof Vests and Stopping Power.

> Essentially, there seems to be three main groups of thought:

> 1 - Because of the physics rule - Every action has an equal and
>     opposite reaction - no person can fire a gun that can knock down
>     another person without themselves being knocked down.  And since
>     the average person doesn't get knocked down when they shoot a gun
>     therefore a gun lacks the power to knock another person down.

Not quite - a person firing a gun braces against the recoil, whereas
a person getting hit by a bullet usually wouldn't.  Essentially, this is
fairly accurate.  There is also recoil-damping to consider, but there are
limits on how effective this can be.

>     One example offered as proof was this.  An individual said that he
>     shot an air conditioning unit with his 30-06 rifle and, though the
>     bullets did not penetrate all the way through thereby dumping its
>     entire energy into the a/c, the a/c just shook a little.  Then he
>     walked over and kicked the a/c over with his foot.  Therefore, he
>     "proved" that his foot has more "knock down" power than his rifle.

Yes - a kick *does* generally have more 'knock down' than a rifle round.
The energy of the rifle round is much higher, though - energy and 'knock
down' (momentum) are different quantities.  The momentum is given by
    P = mv,  where P is momentum, m is mass of bullet, v is its velocity.

I expect a typical rifle bullet would have a momentum of about 1.5 Ns
(newton-second) for a 5 gram bullet at subsonic speed, to 15 Ns for a
10 gram bullet at Mach 5.

(A hard kick would deliver around 50-100 Ns)

> 2 - News stories, police reports, etc are filled with accounts of people
>     being shot with high powered guns and being knocked down and/or 
>     thrown back several feet.  Obviously, guns are capable of ernormous
>     knock down power.

Have you ever grabbed a live wire and been 'thrown' across the room?
It's the same principle - your own muscles propel you.  Because you aren't
in control of this reflex, it feels like an outside force.

> 3 - Though a bullet proof vest may stop the penetration of a bullet
>     and even spread enough of the impacting force so as to prevent
>     internal injuries, the impact from such an event will at the least
>     knock the wearer down or off his feet.  A 30-06 rifle fires a bullet
>     of approx 150 grains (9.72 grams) with a muzzle velocity of approx
>     3000 fps and muzzle energy of approx 2900 ft/lbs.

The energy is immaterial - it's the momentum that matters.  In this case,
the momentum is about 9 Ns - enough to change the target's balance (they
might have to step back) but nowhere near enough to knock them completely
off their feet.

> 1 - How much energy does it take to knock the average person off
>     their feet?

Energy does not come into consideration - most of it becomes heat.
The *momentum* required would be around 50-200 Ns.
(Your momentum when walking is about 100 Ns)

> 2 - What's the difference, if any, from someone being hit in the
>     chest with a sledge hammer or, while wearing a vest that prevents
>     penertration, being hit in the chest by a bullet?

The momentum of a sledge-hammer could well be around 100 Ns.  The bullet
has around 5 Ns.  So - the sledge-hammer would do a *much* more effective
job of knocking you back.  (But the bullet has more energy)
--
Timothy D Little   a.k.a.   u894793@bruny.cc.utas.edu.au