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Comment by brohee | Hacker News Reader

A 9mm round (most typical in John Wick) is around 600J of muzzle energy (and John Wick fights are extremely close range so we can use the same estimate for impact). That's in the range of a strong punch by a good boxer (a good heavyweight can be well over 1000J).

Punches don't ragdoll humans and neither should bullets stopped by a ballistic vest.

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on the other hand, a bullet isn't stopped by bullet proof fabric, it's stopped by what's behind it. If I take a 9mm round to the arm then my arm is breaking. That 600J is focused on a much smaller area than a heavyweight punch.

Yea this. Now conceivably such a sci-fi suit might have a non-Newtonian reaction to a bullet impact and automatically spread the force over a wider area in a somehow non-destructive way.

Dunno about sci-fi, but some tactical gear is incorporating D3O today (see https://www.deltathreeoscar.com/). And I suspect the most advanced stuff is not quite public.

No, that’s the magic of the bulletproof fabric. Imagine the energy of the bullet is distributed across the entire garment.

I think that is our problem, we have a hard time imagining how the energy could be distributed across the garment.

You could sell me on Non-Newtonian-Kevlar though: Pliant to a slow crease, rock-hard with a sudden strike.

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Oh man I thought I was making that up, but it's a thing! https://en.wikipedia.org/wiki/Liquid_Armor The future is now!

It also doesn't have to spread it out over a particularly large area. If we are talking about the force of a punch from a good boxer, we only have to spread it over the area of the front of a boxing glove. That seems perfectly reasonable for a fabric layer soaked with a non-Newtonian fluid.

> You could sell me on Non-Newtonian-Kevlar though: Pliant to a slow crease, rock-hard with a sudden strike.

"Nanomachines, son. They harden in response to physical trauma."

According to that page, BAE systems et al were using kevlar soaked in a solution of polyethylene glycol and silica nanoparticles.

All readily available online, if you can afford something like $200 per 2ml of silica nanoparticles. It would be amazing to see one of the YouTube channels that has tried making their own, just make the real thing.

I imagine a suit made of dripping wet Kevlar wouldn't be too comfortable though. John Wick would experience some chafing.

If the physics is to be believed then it should be roughly the same as being punched in the arm by a strong guy. So long as you aren't suffering from osteoporosis you should be able to take the hit. The trick with bulletproof materials is to spread the energy out, the fabric would need to be like a super oobleck where it becomes completely rigid when hit by a bullet and then return to being flexible afterward. If it is bulletproof but still acts like fabric then the bullet will just pull it along as it pierces through your body.

Agree on the ragdoll thing, but 600J spread out over 50cm^2 is different than than concentrated on 0.75cm^2.

Even if it stopped the bullet, id assume it could break ribs.

Where did OP mention that a human should be ragdolled by a bullet?

Also, I'd love to see you take a punch by a good boxer and remain even remotely still.

A punch from a good boxer into an area smaller than a a boxer's fist, frequently without any kind of preparation or bracing. Yeah it shouldn't ragdoll someone but that seems far more impactful than portrayed.

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