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The design of this grappling hook is to fire hooks at a target at a max distance of 8 meters (~26 feet), a human for ease of calculation, pull the human to about 1 meter (~3 feet) to the person who is wielding the hook. Then the hooks can be quickly removed (under two seconds) from the human even if the human is struggling against the wielder.

Note the hooks are designed specifically for this purpose. A couple of question of the design:

  1. How much force is required to fire the hooks?
  2. Can the hooks ever be able to grab into an unarmored human and a human wearing kevlar armor, and not be removed until the right time?
  3. Are there materials of the rope or chain withstand that amount of force?

For now we will not consider the effects of the firing force on the wielder's arm, or whether the wielder could pull the human that close. We are also not concerned about practicality. Potentially lethal force is fine, but not preferred.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – James Jan 10 '18 at 21:24
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You may want to investigate in some non-aggressive alternative to harpoons.

Depending on the technological level of your world (you mentioned Kevlar, so I assume we're not in a Medieval setting) a nice alternative could be synthetic Setae that have a large adhesive force on almost all solid surfaces (Teflon being one of the few exceptions); this adhesive force can be "switched off" almost instantly by varying the incidence of the nanostructures making up the adhesive surface.

As for your questions:

  1. Force depends on speed you want to impress to your "grappling hook"; the proposed solution is quite lightweight, so you can think about something akin to pitching a baseball.
  2. Yes, it would stick to almost anything, including bare skin. Excessive sweat or other oily coating could reduce considerably adhesive force. You can counteract enlarging surface (think ball inside a wet towel).
  3. Force you need to express is the force needed to drag an unwilling human. You may easily have a Kevlar line strong enough and a small electrical winch, but you also need an anchor to pull your "prey" to you, otherwise you risk being dragged to him. You might want to embed some taser-like device to temporarily inhabilitate target.
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  • $\begingroup$ This actually fits pretty well! Good answer. $\endgroup$ – OneSurvivor Jan 8 '18 at 22:27
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    $\begingroup$ This type of weapon can be easily negated if its use is expected - targets would just have to hang toilet paper as the outer shell of armor. $\endgroup$ – Alexander Jan 8 '18 at 22:58
  • $\begingroup$ @Alexander: true, but that would hardly withstand any serious fighting for more than a few seconds. The good thin is this grapple is reusable. $\endgroup$ – ZioByte Jan 8 '18 at 23:37
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If you're looking for this:

Twilight Princess Clawshot

Then your answer is No, that's not possible.

But if you're just trying to see if you can pull a human into melee range using a hook... Of course you can! With enough power, you can do anything!

Let's take a look at the physics... The average weight of a human is 62kg (137lb). Given that you said melee range, I'm gonna assume that they're and they are wearing armor. We'll round them to 80kg. Now, we can start at the end. Assuming they have an embedded hook in them... Could you reel them in? Math time! We can see that, by their mass, their normal force is $9.81\frac{m}{s^2}*80kg= 784.8N$. Given a coefficient of friction of .4 (I totally fudged that number, but it's within acceptable ranges), you'd need 314N of force to pull your attacker to you on level ground. Given that you're pulling them 7 meters (As stated above) the energy you'd expend is 2198J, or about a half a food calorie. That's okay, it's small, in fact. XKCD has a delightful article on tug-of-war and related things here, which is what this ends up being. You've gotta pull with more force than your opponent AND not move, yourself.

Now, let's analyze the firing mechanism. We know that the claw mechanism needs to be able to withstand up to 1000N of force(314N *2 + some rounding to account for heavier opponents.) If we assume a relatively small 'tug' area of $15cm^2$, then we can see the pressure on the 'wound site' is 66Pa. It'll hurt, but it probably won't rip out. You could use a hook mechanism like that of a harpoon for the engage/disengage mechanism.

So... Why couldn't you just go ahead and use a harpoon?

Having read two whole scenes from Moby Dick, I recall that the harpoon was mainly used to keep ahold of the whale and to bleed it out until it died. Even though whales are much bigger than humans,Citation needed the forces at work are similar. The XKCD article I linked earlier says that the average pull force of 'elite tug-of-war players' is around 1000N (Well...actually it uses kilogram-force, but I converted it.) In other words... No matter what kind of device you're using, it's gonna be at the limit of your capability to pull your opponent to you.

Moving on to kevlar...

Kevlar vests are designed to stop bullets. A quick google says that the average force of a bullet is 304 N. Kevlar is stronger. Furthermore, any hook you might use would have a bigger cross section than a bullet. Ergo, kevlar beats harpoon.

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    $\begingroup$ This is a good answer, but I have to disagree on the kevlar comment. Kevlar vests that can stop bullets are notorious for failing to stop knives, since Kevlar fibers are great at stopping blunt, high-speed, low-momentum projectiles, but not sharp, low-speed, high-momentum ones. A harpoon is probably closer to a bladed weapon than a bullet in how it interacts with armor. $\endgroup$ – Catgut Jan 9 '18 at 16:31
  • $\begingroup$ Interesting! I do admit, I'm not really an expert on any of this, and about a third of my physics knowledge is learned on the spot. I'll cross-validate your Kevlar claim, then update my answer when I've got time. $\endgroup$ – Jakob Lovern Jan 9 '18 at 16:45
  • $\begingroup$ Your picture is broken $\endgroup$ – Aify Jan 9 '18 at 18:05
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Yes; your modified grappling hook is basically a harpoon gun with a detachable tip for ease of removal.

To answer your sub questions:

1) Depends on the size of the harpoon

2) Yes. With regards to kevlar armor though, depends where you hit and the power of your harpoon gun.

3) Yes. Plenty, steel cable seems to work just fine. Alternatively, you can use kevlar line.

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Have you ever tried to drag someone, or push him?

When you do that you notice something: it requires a lot of strength to even move him from a step, it's easier to lift him: look at nearly all riot police arrestation. They prefer to lift the guy than to drag or push him, because they don't need as much strength to proceed and he can't do much to resist.

And what you are requiring is a weapon that you could carry and use it to drag somone for over 7 meter? Even if you are way stronger than your opponent it would be a very long and hard task.

And, even if that work, what could be the purpose of such a thing? Riot police are way more effective as they are possible with today's technology. With that range there is a high chance that some people will try to pull the chain to make you release your grip.

There are so many flaws in the concept itself that there is no way it would work.

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