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If a sword were to be swung horizontally, through the torso of a human being, how fast would it need to be to not have blood on it after the swing?

Assume the sword and its user are together able to cut through all of the proper bodily organs and bones in a single strike. Assume a well-forged steel blade.

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    $\begingroup$ What is the era for this event? $\endgroup$
    – Joe Kissling
    Commented Apr 9, 2017 at 3:59
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    $\begingroup$ Fret not all, I have adjusted my answer to cover you concerns. $\endgroup$
    – Joe Kissling
    Commented Apr 9, 2017 at 18:59
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    $\begingroup$ Is the subject standing in any sort of shower, lathering themselves with soap? $\endgroup$
    – moopet
    Commented Apr 10, 2017 at 9:07
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    $\begingroup$ People used to grease their blades for that purpose. $\endgroup$
    – njzk2
    Commented Apr 10, 2017 at 16:19
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    $\begingroup$ I feel like this question could only be answered appropriately by an xkcd. $\endgroup$
    – GeneralMike
    Commented Apr 10, 2017 at 18:43

7 Answers 7

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Speed Not Required

Give your sword a hydrophobic coating and it will never get any blood on it. The coating repels liquids, causing them to bead up like water on a lotus leaf and just run off. Bonus it stays nice and clean.

Edit:

A blade made of steel does not mean that It cannot have a coating that makes it repel blood. Furthermore, the surface geometry of the blade may be altered to give the steel hydrophobic proeprties. This laser can do it to a variety of metals.

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    $\begingroup$ coating are generally very sensitive to scratches... cutting through a bone is going to give some scratch, or not? $\endgroup$
    – L.Dutch ♦
    Commented Apr 9, 2017 at 7:15
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    $\begingroup$ @L.Dutch I don't know how far it was pursued, but I heard several years ago about work being done on an ultra-hydrophobic surface which was also self-healing, based on a design inspired by the cellular structure of pitcher plants. So a durable hydrophobic coating should be possible, or at least plausible. $\endgroup$
    – Dave Sherohman
    Commented Apr 9, 2017 at 8:15
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    $\begingroup$ There's also a technique that laser etches a surface to make it hydrophobic. $\endgroup$
    – T.J.L.
    Commented Apr 9, 2017 at 16:01
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    $\begingroup$ It's the best option, but blood still adheres. I have done research at one of the major companies for dialysis and it's a big issue with how to clean dialyzers and tubings for clinics which wishes to save money by re-using equipment. Yes, the best way is to make it hydrophobic, but it's impossible to make a surface where no parts of the blood adheres. $\endgroup$
    – Mrkvička
    Commented Apr 9, 2017 at 19:08
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    $\begingroup$ @Dan Neely "Perfectly", "forever", and "without maintenance" were not stipulated requirements. $\endgroup$
    – T.J.L.
    Commented Apr 10, 2017 at 17:47
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Not only is speed not required as per Joe Kissling's answer but speed is not able to keep it clean, either. The sword will be pushing through blood as it cuts, it's impossible for blood not to get on the sword.

Thus keeping it clean means either a material that blood will not adhere to, or a speed in which the air movement will scour the blade free of blood.

Consider airplane wings--even going hundreds of miles per hour is not enough to sweep ice from the wings despite a smooth surface. Ice doesn't bond all that tightly. The airline industry combats it by spraying on a coating (note that the coating also doesn't get blown off) that lasts long enough for the plane to get above the threat zone.

While this doesn't give us an exact answer it shows that we need a speed far beyond what muscles can deliver. My gut says the required speed is supersonic (which means a nasty sonic boom from the sword) but I do not know.

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    $\begingroup$ Related question: how fast must I move my sandwich thru this stream of filth to avoid getting any filth on it. There is no speed at which I will still be interested in eating that sandwich. $\endgroup$
    – Willk
    Commented Apr 9, 2017 at 5:07
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    $\begingroup$ You cannot clean surfaces with speed because directly at the surface boundary every molecule clings to the surface molecules due to internal adhesive forces. Combined with the fact that nearly every fluid has internal viscosity it leads to the typical speed profile of every fluid standing still at the surface and gradually increasing speed with increasing distance. Every smooth surface will have a very, very thin water layer on it which is extremely tenacious. For ultra-high vacuum we must get rid of this layer, meaning heating the surface up to 300 (!) °C. $\endgroup$
    – Thorsten S.
    Commented Apr 9, 2017 at 10:32
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    $\begingroup$ "speed is not able to keep it clean" - it is, if high enough. At very high speeds, the blood will boil away, at even higher speeds, the steel of the sword itself will boil away. And close enough to light speed, the very fabric of reality around your sword will boil away :) $\endgroup$
    – vsz
    Commented Apr 9, 2017 at 10:53
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    $\begingroup$ As @vsz says just check what-if.xkcd.com/1 $\endgroup$
    – DRF
    Commented Apr 9, 2017 at 11:54
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    $\begingroup$ @vsz Actually, at any speed the fabric of reality is still pretty much intact. It's accelerating / decelerating that you do that might stress it somewhat, especially if you're massive. See gravitational waves. $\endgroup$
    – wizzwizz4
    Commented Apr 9, 2017 at 18:24
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It's the stopping speed you're after. Say the swing speed is a bit better than a golf club's speed swung by a pro-golfer. We're looking at 100 mph or 44 m/s.

If your hero/villain brings the sword to a standstill after the cut in half a second, the g-force will be ~9 Gees. I'd say it's enough to propel the blood off the sword (testing required).

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    $\begingroup$ Unlikely. Blood is a liquid with adhesion. Consider that golf club. You hit the ball it dewy grass. The ball flies away. The bigger drops of water fly away. But the club head is still wet. I sincerely doubt that any breaks applied by human muscle will get the sword clean. $\endgroup$
    – SRM
    Commented Apr 9, 2017 at 14:22
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    $\begingroup$ That is not too far off, actually. I learnt some japanese samurai sword fighting back in martial arts days (Iaido). All of the katas end with "shaking off" motion, that is basically a fast swing to the side/down rapidly stopped. Since we didn't train with any living humans, I can't say if it is really effective at throwing off the blood, but Iaido is not exactly known for having any useless moves. $\endgroup$
    – Tom
    Commented Apr 9, 2017 at 16:33
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    $\begingroup$ @Tom it would remove the bulk of the blood, but it wouldn't clean the blade. I have been working with blood and I know from experience that the only two ways to get it off a surface is 1) wiping/scraping (enough with a damp cloth if it's still fresh blood) or 2) rinsing it with (a lot) of detergent and water. The Iaido moves are either to remove the bulk or simply a ritual move, a swordsman would still have to clean the blade after battle. $\endgroup$
    – Mrkvička
    Commented Apr 9, 2017 at 19:00
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    $\begingroup$ @Tom To further iterate on Mrkvička's comment, it is useful for getting the bulk of the blood off, but there would still be some blood left. It was primarily used to keep a cutting edge relatively clean to finish the battle easier, but it never replaced a wipe-down with a fine cloth. (If you want to be showy about that, then put the cloth in your elbow or shoulder, and run the sword through it. Still badass, but practical, too!) $\endgroup$
    – JessLovely
    Commented Apr 9, 2017 at 19:23
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    $\begingroup$ Both answers are probably right. Just to complete the picture: In Iaido this move is done immediately before sheathing the blade. That means it would be sufficiently clean to be put away. The Samurai would certainly clean it properly once safely home. Example, here's Mae, the first Iaido kata: youtube.com/watch?v=sDfcu9BP5fI $\endgroup$
    – Tom
    Commented Apr 10, 2017 at 4:59
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To elaborate on anon's answer, there is a motion called Chiburi, excercised by the samurai. In movies it is a quick flick of the sword that removes blood, but in reality the sword still requires extensive wiping with a piece of cloth.

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    $\begingroup$ There are a number of comments on anon's answer that cover this. Mrkvička said, it would remove the bulk of the blood, but it wouldn't clean the blade. I have been working with blood and I know from experience that the only two ways to get it off a surface is 1) wiping/scraping (enough with a damp cloth if it's still fresh blood) or 2) rinsing it with (a lot) of detergent and water. The Iaido moves are either to remove the bulk or simply a ritual move, a swordsman would still have to clean the blade after battle. $\endgroup$
    – Draco18s no longer trusts SE
    Commented Apr 11, 2017 at 12:33
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Okay I tried to relate kinetic energy of the sword with the temperature needed to boil blood (which would mean that the blood boils off the sword as it's cutting through the victim, and hence no blood on the sword). I don't know if I did this correctly, as I couldn't find a nice way to relate kinetic energy and temperature, but I found that the sword would have to be going 409 m/s (or 914 mph).

Edit: Here are my equations used...

$<KE> = <\frac{1}{2}mv^2> = \frac{3}{2}kT$

Which is average kinetic energy equals the average of one half the mass of an iron atom times its velocity squared. Then that equals three halves of the Boltzmann constant times temperature. I used:

  • $m$ = 9.27E-26 kg (atomic mass of iron (since swords are mostly iron))
  • $k$ = 1.38E-23 J/K (Boltzmann constant)
  • $T$ = 374 K (temperature at which water boils plus a little bit to account for the salt in blood which raises its boiling point to about 374 K)

Plug everything in and solve for $v$. That yields $v$ = 409 m/s. To see what that compares with in terms of the sword as a whole's kinetic energy, I used the kinetic energy equation again, only this time using this velocity times the sword's mass ($m$ = 1.4 kg, mass of a katana). I got 117,000 Joules of energy. Which feels kind of low to me - that's like a car hitting you on the road - but maybe I'm not relating these equations correctly.

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    $\begingroup$ Can you add your equations to your answer? $\endgroup$
    – Joe Kissling
    Commented Apr 11, 2017 at 5:12
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    $\begingroup$ sounds like you would turn the sword into a light saber :p $\endgroup$
    – Sarfaraaz
    Commented Apr 11, 2017 at 8:05
  • $\begingroup$ @JoeKissling Yes I'll find the paper I wrote it on and add equations later tonight. $\endgroup$
    – magnetar
    Commented Apr 11, 2017 at 22:13
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    $\begingroup$ This is a good attempt at using math, but you aren't comparing these things correctly. The KE that you can relate to temperature is the root mean square KE of the particle itself in the stationary frame of its surroundings. That is, for a pool of blood at rest, your individual water molecules in the blood need a RMS velocity of 409 m/s in order to evaporate. However, for a swinging sword, the air in contact with the blood would also be accelerated to near his speed due to the mechanics of the laminar flow of air around the blade. Thus, the velocity of the blood does't help you. $\endgroup$
    – kingledion
    Commented Apr 19, 2017 at 23:47
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The speed of a sword can, in fact, affect how much blood is left on it. But for a sword to have no blood on it whatsoever, the sword would have to be faster than the speed of the blood dripping onto it while it's in the human body. the only speed at which this is possible is the speed of sound. 334 meters per second. If the sword breaks the sound barrier 1 millimeter before hitting the target it would rip the body and the sword would only continue behind the massive force of a broken sound barrier. The sword would simply act as a device to break the sound barrier and the use the explosion to cut the target in half. If any human could achieve this they may as well use something else made of steel like a crowbar or a steel pipe. So if you want to split someone in half and not get the tool you use dirty. Acquire superhuman strength and break the sound barrier next to your victim. (i think I made a new superhero ).

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OK, a theory that might actually be close:

Assumption: All the blood is equivalent to a mass of water.

Further assumption: If struck above speed of sound in water (which is ... high!), anything striking it will not be good at selectively displacing part of it to make way, and try to accelerate the whole mass, which will appear close to one solid object, which gets effectively shattered into two (heavy and hard to accelerate) or more (flying off at an angle) pieces in case you manage to cut through it.

Problem: It is unlikely that a metal edge will stay intact after that kind of impact.

Since the displacement needed for a blade to pass is very small compared to eg an unlucky person hitting a body of water flat, a lower speed impact is likely to displace some of the liquid as a liquid and accelerate it together with the cutting edge, staying on it.

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  • $\begingroup$ Sorry, but I don't understand what you mean here. $\endgroup$
    – Mołot
    Commented Apr 20, 2017 at 23:17
  • $\begingroup$ Mostly about "if it is possible, it is by holding Newton to his promise about objects at rest tending to stay so". I'm not knowledgeable about the maths really... $\endgroup$
    – rackandboneman
    Commented Apr 20, 2017 at 23:21

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