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Old 02-23-2017, 03:20 PM   #1
Skarg
 
Join Date: May 2015
Default (Bullet Knockback) What's wrong with this picture?

So at some point, GURPS recanted it's original position that bullets could cause knockback. I've seen dozens of forum discussions, here and elsewhere, professing how it shouldn't be more than the recoil and so no one should be knocked backwards.

So is this video wrong/fake, or not imply that a bullet could cause some degree of knockback?

Last edited by Skarg; 02-23-2017 at 03:21 PM. Reason: putting actual subject in title
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Old 02-23-2017, 03:28 PM   #2
Mathulhu
 
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Default Re: (Bullet Knockback) What's wrong with this picture?

Assuming the target and the shooter are approximately equal in mass then the target will experience the same amount of "knock back" as the shooter.
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Old 02-23-2017, 03:36 PM   #3
Bruno
 
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Default Re: (Bullet Knockback) What's wrong with this picture?

Quote:
Originally Posted by Skarg View Post
So is this video wrong/fake, or not imply that a bullet could cause some degree of knockback?
That is not a 150-200 lb block of gelatin. If it's even 20 pounds, I'll go buy a hat so I can eat it.

If you shoot a cat with a .45, the cat will not be in the same place it started. That block is somewhere in size between a cat and a modest dog.

EDIT: or to answer your question directly: It is neither fake, nor does it imply that a bullet could cause some degree of knockback to a person-ish sized target. Nobody argues that you can't shoot an empty soda can and send it flying.
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Old 02-23-2017, 03:37 PM   #4
Polkageist
 
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Default Re: (Bullet Knockback) What's wrong with this picture?

Correct, that video does not imply that a bullet would/could cause knockback.

The block of gel has got to weigh at most 10lbs, so like < 1/10th of a person, and the bullet didn't even knock it off the stump. A bullet hitting a Standard Adult Human would barely shift that much mass.

It'd do all the other nasty stuff bullets do, but shoving the whole person around isn't really one of them.

edit: To put more physics explanation to it, the 'pushing' force (sorry, I'm gonna use some slightly non-scientific terms here) is shockingly low. It is, really, the amount of force felt in the recoil. Bullets, moving very fast and having a very small surface area allowing them to pierce things causes all sorts of damage due to shockwaves, penetration, etc., etc.

I haven't done the math but I would wonder how a 'bullet' which has the same mass (a few grams) but with a surface area of a square foot would act when it hit something. Ignoring things like air resistance, means of acceleration, stuff like that.
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Old 02-23-2017, 03:57 PM   #5
Skarg
 
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Default Re: (Bullet Knockback) What's wrong with this picture?

Hmm, ok. I know it's not that heavy a block and it didn't fly back a yard, and that is a hollow point bullet.

On the other hand, it was a long block lying flat and it got lifted up in the air and was held back by a notch in the stump. There was definitely some force imparted. I was surprised by how much it did move the block.

I can see it wouldn't be fully forcing people back the way the old mechanic worked, because yes conservation of momentum etc. But on the other hand, people who fall down and around from getting hit and shoved in real life don't need to send others flying away either, as standing people can lose control of balance from being hit and shoved. So for those of us with house rules for chances of random falling about sometimes not requiring a whole lot of damage, it seems interesting to see this.

Thanks for your thoughts. :-)

Hmm, actually, on looking at it again, it looks like maybe the bullet didn't cause much/any knockback, but the cavitation collapse & explosion of air caused gas to shoot out, which did.
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Old 02-23-2017, 03:58 PM   #6
fula farbrorn
 
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Default Re: (Bullet Knockback) What's wrong with this picture?

As others have stated, bullets will not move a person around and here is a video showing just that

https://www.youtube.com/watch?v=o5f1Fo4r4_I

Also bullets do almost nothing through a vest designed to stop them, Behind armour trauma is often negligible At gurps depth of resolution
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Old 02-23-2017, 04:36 PM   #7
Anthony
 
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Default Re: (Bullet Knockback) What's wrong with this picture?

Realistically, bullets do cause a little bit of knockback; it's just well below the resolution of GURPS hexes in any normal situation (a modest size cannon would knock a human back by a yard or more -- or more accurately, the average position of the scattered remains would be a yard or more back -- but this is unlikely to matter outside of a superhero game).
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Old 02-23-2017, 09:49 PM   #8
Bilanthri
 
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Default Re: (Bullet Knockback) What's wrong with this picture?

This video does, however, show a great example of the damage caused by hydrostatic shock. All that energy crushing cells around the wound channel, and then the wound channel expanding and collapsing, is what really makes a bullet wound more than just a high-velocity impalement.

Cool video for the science-minded.
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Old 02-23-2017, 11:17 PM   #9
Ulzgoroth
 
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Default Re: (Bullet Knockback) What's wrong with this picture?

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Originally Posted by Bilanthri View Post
This video does, however, show a great example of the damage caused by hydrostatic shock. All that energy crushing cells around the wound channel, and then the wound channel expanding and collapsing, is what really makes a bullet wound more than just a high-velocity impalement.

Cool video for the science-minded.
Except 'crushing cells' with shockwaves isn't terribly effective (people don't suffer massive necrosis from either bullet wounds or explosive blast-waves) and most tissue is elastic enough to snap back from the deformation...
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Old 02-23-2017, 11:36 PM   #10
Tomsdad
 
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Default Re: (Bullet Knockback) What's wrong with this picture?

Quote:
Originally Posted by Skarg View Post
....

I can see it wouldn't be fully forcing people back the way the old mechanic worked, because yes conservation of momentum etc. But on the other hand, people who fall down and around from getting hit and shoved in real life don't need to send others flying away either, as standing people can lose control of balance from being hit and shoved. So for those of us with house rules for chances of random falling about sometimes not requiring a whole lot of damage, it seems interesting to see this.

....
Thing is being shoved by another person is an awful lot more force (capital F) and momentum over a relativity long period of time, than being shot by a hand held weapon that firing a very small and very fast projectile.

And of course a lot of movement upon getting shot is the person's reaction to the trauma of being shot (as opposed to the physical impact of it) which would be part of the failed HT rolls and knockdown. I'm sure actual knockdown reaction involves some horizontal movement as well as vertical!

Last edited by Tomsdad; 02-24-2017 at 05:48 AM.
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