Can a Corset Stop a Knife?
Added 2020-10-05 11:59:43 +0000 UTCDearest patrons, I have Made an Extra Thing, because I saw Enola Holmes last week and I just had to do the research: could a corset actually stop a knife?
Turns out...maybe.
So before I release this episode into the wild, enjoy! (And maybe check my assumptions to make sure there’s nothing too obviously wrong there)
Comments
I quite like this! "Sponsored by Ankle Biters, the only guaranteed snake-proof sock!"
Jill Bearup
2020-10-07 06:49:11 +0000 UTCThere are 1,000,000 square mm in one square meter, so a 1 sq mm crack takes only 0.018J. Whatever that means!
Dr Martin Ward
2020-10-06 14:37:49 +0000 UTCThis was great. You should do a whole series based on underwear stopping things. Can knee-socks stop a snake bite? Can a jockstrap stop a crossbow bolt? Can a bullet bra stop an octopus fired from a drainpipe?
GammaWu
2020-10-06 13:53:16 +0000 UTCThe people in the paper calculated that it takes 18kJ of energy to make a 1 square metre crack in baleen, which is to say, 18kJ/m^2. Which was where that number came from. I have significantly shortened the calculations for the final version of the video though, and am probably taking that one out (and putting in what the assumptions are and mentioning that they're kind of squirrelly :D) Thank you so much for your help!
Jill Bearup
2020-10-06 13:17:38 +0000 UTCThe maths is fine: apart from the 18kJ/m calculation (see my message above)!
Dr Martin Ward
2020-10-06 10:27:17 +0000 UTCAnother thought: for applying force to a moving object, Power = Force x Velocity. Given the maximum power that can be expended by the attacker's arm the force is limited to Power/Velocity at the moment of impact. Arms can supply less power when pushing compared to pulling (which is why rowers pull on their oars). The motions of a person's arm can generate up to 60 watts https://www.knowablemagazine.org/article/technology/2018/body-electric (pushing will presumably generate less power than this). If Enola manages to move back at 0.8m/s (as you estimated) then the force is limited to 75N: much less than the 200N that can be applied to a stationary target. Skin penetration typically requires 35-55N, so given that the corset is definitely much tougher than skin, I think it can resist the force. Silk is also a fairly strong material which could at least dissipate some pressure over a larger area. Note: the 18kJ figure for baleen is 18kJ per *square* metre. I don't know how to translate that into the force needed to pierce baleen with a sharp knife!
Dr Martin Ward
2020-10-06 10:20:34 +0000 UTCMy main problem is that you missed an opportunity for "You should have gone for the head" joke.
Anders
2020-10-05 19:24:46 +0000 UTCI think I may have to cut down on the maths a little bit :D It just took so darned long to work out that I wanted to include it all! But honestly I can probably do more on-screen equations and less explaining, since what people are looking for is yes or no or maybe, and most people won't care about the numbers.
Jill Bearup
2020-10-05 18:20:10 +0000 UTCI'm formally trained as an analytical chemist and have to admit my eyes glazed a bit on the 'maths' - but the approach makes sense. I would have been satisfied with the 'anecdotal' - which in truth, may be underselling. If it was reported and researched (by say police), it could be seen as 'fact'. n=few does not mean it's invalid, just that it may not be representative - say more than 2-3 sigma out.
Firstspear
2020-10-05 17:44:31 +0000 UTCThe force the stabber applies to the knife varies with distance, so by moving back Enola's body contacts the knife in a position where the stab is weaker. The force is also a combination of muscle force applied plus the force required to decelerate the fist. The latter force can be reduced if the distance over which it is applied is increased. Also (I just noticed) by moving back, Enola has time to grab the attacker's hand and apply some opposing force.
Dr Martin Ward
2020-10-05 14:22:00 +0000 UTCClearly, I need to make the time to watch this movie. (I don't mind having been spoiled on "there was a knife used in one scene," after all.)
Karel P Kerezman
2020-10-05 14:14:37 +0000 UTCTrue, but it's still the same body being moved through space, right? So does it matter how the force is absorbed, assuming it's the same force being absorbed one way or the other (i.e. into Enola's body?) ETA: wait, no, I get you. I need to put “assuming perfectly elastic collision” (which of course it wouldn’t be, but overestimating the force seemed better for a first approximation)
Jill Bearup
2020-10-05 14:01:02 +0000 UTCI think you missed something: Enola is in a fist fight and is skilled at fighting. She thought that the knife strike was a fist strike, so she moved back to "ride" the blow. This would probably absorb about half the force.
Dr Martin Ward
2020-10-05 13:07:39 +0000 UTC
