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Comment by m_a_g | Hacker News Reader

My biggest problem with those imaginary suits is the transfer of energy.

Even if a lightweight bulletproof material like that can be made, there needs to be some symptom of energy transfer visible to the eye. Otherwise, it's not sci-fi; it's magic

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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.

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.

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.

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.

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.

That's my main gripe with how the bulletproof suits were shown in the fourth movie. There is a difference between getting shot in the chest and actually catching a bullet with your suit while using it as a shield. The former is believable because the kinetic energy would dissipate in your ribs (possibly breaking them) the later isn't because it's roughly equivalent to dissipating the bullet's energy using your arm.

OTOH it's a John Wick movie, expecting anything to be very relatistic is already thinking one step too far.

I haven't mathed this out, but from experience from jumping (both from heights and towards ledges) I would think that our arms and legs are much better shock absorbers than our ribs. If you hold something like a shield, you are primarily using your triceps and shoulder muscles as springs to dissipate the impact over the distance of about a foot. Of course there are issues with that, but it's much better than your rips which can move much less before breaking.

No worries, it's a Culture suit with a built-in trapdoor system (Iain M. Banks)

> "Trapdoor system: A mechanism that transports any unwanted energy inside of the ship into hyperspace, so it can't be destroyed by internal attacks. It was stated that you could set off an H-bomb inside of a Culture ship if you were right next to it, and all of the energy would be instantly transported into hyperspace so it would seem like nothing had happened at all." (outskirtsbattledomewiki)

Good thing John Wick isn't scifi then. It'd be like complaining that the Predator has elements that aren't realistic when it comes to physics: it's true, but it's not exactly the point of the movie to make you think deeply about physics, is it ?

In the 2nd movie (and perhaps other places), he gets shot repeatedly with rifles. Even if the shots don't penetrate, the repeated impact alone would break ribs and leave him incapacitated.

It's suggested that he is having ribs broken, or at least badly bruised, by every major hit

I think that could still kill as well. Repeated shots break the ribs, and a subsequent shot pushes a broken piece of rib into the lung causing bleeding into the lung or maybe pneumothorax.

Either that or the kevlar failing due to repeated shots. It's not meant to be shot in the same place more than once.

Still more believable than Matrix style bullet dodging, though.

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