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Re: [Phys-L] Underwater AK47 shot in slo-mo



On Aug 3, 2013, at 10:48 PM, Bob Sciamanda <treborsci@verizon.net> wrote:

Underwater shot with some analysis:

http://www.youtube.com/watch?feature=player_embedded&v=cp5gdUHFGIQ

1) I don't understand everything I'm seeing in the video.

2) The presenter on the video doesn't understand it either.

*) For one thing, almost everything he says about Bernoulli's principle
is wrong. It's an extreme example of equation-hunting. The principle
itself is not wrong, it's just being applied wrongly. You cannot use
it to compare the pressure of some parcel "here" with some other parcel
"there", unless you know a_priori that the two parcels started out with
the same head ... which is flagrantly not the case here.
http://www.av8n.com/physics/bernoulli.htm
especially
http://www.av8n.com/physics/bernoulli.htm#sec-mutations

On 08/04/2013 08:32 AM, Chuck Britton wrote:
Someones gonna hafta convince me about this 'Cavitation' in the
barrel in FRONT of the bullet.

A more likely explanation (to MY weak eyes) is that this is the small
amount of gas that leaks around the bullet as it is accelerating down
the barrel.

*) Agreed, it is fairly hard to believe there could be cavitation in
front of the bullet within the barrel.

There will be a jet of high-speed water coming out of the barrel
ahead of the bullet. After the jet of water has left the barrel
(not before!) it could perhaps produce cavitation ahead of the
bullet. I'm not saying I know what happens, but I know enough
not to make predictions about things I don't understand. Eight
times burned, ninth time shy.

The usual rule applies: Everybody thinks they understand fluid
dynamics, but verrrry few of them actually do.

*) If you work hard enough, you can actually concoct a scenario
where there could be cavitation in the barrel. It's a stretch,
but here goes: Suppose the bullet imparts some momentum to the
water ... and then due to friction or whatever, the bullet slows
down. The water then runs away from the bullet. This could just
maybe produce a low-pressure region ahead of the bullet, while
the bullet is still in the barrel.

I don't actually think any such thing is happening in an AK-47 in
this situation, because I reckon there is enough hot gas behind
the bullet to ensure that it is accelerating (not decelerating)
all the way ... especially enough to accelerate it into any low-
pressure bubble formed by the alleged cavitation.

*) The presenter is right about one thing: There are two dissimilar
bubbles, one that contains combustion gases and one that doesn't.

*) I don't think there are /shocks/ involved. The speed of sound in
water at 5 C is 1427 m/s whereas the muzzle velocity of an AK-47
*in air* is commonly quoted as 715 m/s. I don't see how firing
it underwater could make the bullet go faster.