'Shotguns have dkb/dbt' - where did that myth (?) come from?

[Anthony]

Originally Posted by lwcamp

The 7 kg arm of a karate master punching at 7 m/s will have 170 J of kinetic energy and 49 N-s of momentum.

Um...wow. 7 kilogram arms? I think you're vastly overestimating the weight of the arm, particularly the amount of the arm which is actually moving at peak speed; based on how much water I displace when I put my arm in a container of water, the forearm is maybe 1 kilogram. For reference, knife-proof vests are generally normalized for a 40J impact, which is more than most people can actually deliver.

Of course, knocking someone back with a punch also pretty much doesn't happen, unless they fall over from the shock. About the only impacts which realistically cause any level of knockback are collisions or other similar attacks which involve the full body weight behind the blow.

[vicky_molokh]

Originally Posted by Anthony

Um...wow. 7 kilogram arms? I think you're vastly overestimating the weight of the arm, particularly the amount of the arm which is actually moving at peak speed; based on how much water I displace when I put my arm in a container of water, the forearm is maybe 1 kilogram. For reference, knife-proof vests are generally normalized for a 40J impact, which is more than most people can actually deliver.

Of course, knocking someone back with a punch also pretty much doesn't happen, unless they fall over from the shock. About the only impacts which realistically cause any level of knockback are collisions or other similar attacks which involve the full body weight behind the blow.

My biomech is pretty rusty, but IIRC the torso is 40% and arms are 9% the weight of the human normally.

I'm also surprised that a punch of a respectable combatant is less than 10m×s^-1.

[Anthony]

Originally Posted by Molokh

My biomech is pretty rusty, but IIRC the torso is 40% and arms are 9% the weight of the human normally.

Depends on what you count as being the arms, but in any case, that would give 7 kilograms for both arms put together, and at that weight probably includes upper arms and shoulder, which aren't really part of the impact mass of a punch.

[Langy]

A well-delivered punch shouldn't just use the mass of the arm, should it? I'd think a power punch would involve putting the torso behind the blow rather than just the arm - or at the very least, the shoulder.

Then again, I haven't practiced any martial arts in over thirteen years and the only time I punched anything since then I broke one of my fingers, so I'm not exactly an expert.

[Anthony]

Originally Posted by Langy

A well-delivered punch shouldn't just use the mass of the arm, should it? I'd think a power punch would involve putting the torso behind the blow rather than just the arm - or at the very least, the shoulder.

A well-delivered punch involves torso muscles, but the actual impact mass is much smaller. Essentially, a punch uses a mechanism very similar to a whip crack to transfer rotational energy generated in the hips and torso into hand velocity, but the mass which is actually relevant for knockback purposes is only the hand and for some types of strike the forearm.

[Kromm]

Usual estimates for punches assume about 1/6 the striker's mass, if the striker is leaning into the blow. Good punches aren't "whip cracks" that channel tons of energy into a hand. Good punches are closer to battering rams that have the entire arm, shoulder, and upper quarter of the torso behind them. An uppercut can do significantly better.

[vicky_molokh]

Originally Posted by Kromm

Usual estimates for punches assume about 1/6 the striker's mass, if the striker is leaning into the blow. Good punches aren't "whip cracks" that channel tons of energy into a hand. Good punches are closer to battering rams that have the entire arm, shoulder, and upper quarter of the torso behind them. An uppercut can do significantly better.

What would be the in-game difference between the two? One as a Defensive Attack or what?

[Anthony]

Originally Posted by Kromm

Good punches aren't "whip cracks" that channel tons of energy into a hand. Good punches are closer to battering rams that have the entire arm, shoulder, and upper quarter of the torso behind them.

In that case, reduce the '7 m/s' figure to 2-3 m/s, since your torso velocity is going to be around your step velocity and your shoulder velocity isn't going to be dramatically higher. There's a lot of options for how the energy is delivered, but the total impact energy is going to be in the 25-50J range for an average person.

[HANZO]

There have been many cases of police or other law enforcement getting hit multiple times in a vest and having to be treated for internal bleeding from the impact of a hand gun at close range. as well as cracked ribs and a lot of bruising.

I think the myth is that wearing a vest allows the wearer to just keep taking hits like he is superman.

But then again look at the bank robbery in Hollywood a few years back. guys with layered heavy armor. Video

Then again this was what allowed the police there to start carrying AR-15s.

[vicky_molokh]

Originally Posted by HANZO

There have been many cases of police or other law enforcement getting hit multiple times in a vest and having to be treated for internal bleeding from the impact of a hand gun at close range. as well as cracked ribs and a lot of bruising.

I think the myth is that wearing a vest allows the wearer to just keep taking hits like he is superman.

But then again look at the bank robbery in Hollywood a few years back. guys with layered heavy armor. Video

Then again this was what allowed the police there to start carrying AR-15s.

Um, I'm not expecting the bulletproof vest to be bulletproof. But there seems to be an opinion that shotguns do 90% of their damage without penetrating the vest at all through the sheer 'crush' (against which vests supposedly offer no meaningful protection), unlike single-ball weapons which do most of their damage through penetration or 'pierce' (to use GURPS terminology).