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I want one of my characters to be a full blown "ninja".

By this I don't mean an actual japanese ninja mind you, but merely someone who embodies the various tropes that ninjas have become associated with in fiction.

To be more precise I want my character to be lightning fast, incredible nimble/agile, flexible, light as a feather, capable of downright absurd acrobatic and athletic feats.

So I was wondering if the ability to reduce body/mass density (within a certain limit) might be a plausible explanation for such prowess ?

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  • $\begingroup$ Would he be able to decrease the density of certain body parts (arms for example) or just his entire body at once. $\endgroup$ – D.J. Klomp Apr 11 '20 at 11:00
  • $\begingroup$ @D.J.Klomp Initially envisioned it as being the entire body at once but I'm actually open to exploring both of those ideas, what would be the difference ? What benefits would it bring ? $\endgroup$ – Anonymousworldbuilding Apr 11 '20 at 11:04
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I think it will greatly influence the agility and flexibility of a ninja. Basically since you can play with inertia, impulse and air resistance.

If you can change your complete body mass you effectively can change your kinetic energy when you are in motion (E=1/2*m*v^2). This energy will be reduced through air friction, when you for example jump, proportional to the speed squared and area of the object (very simplified). Hence why a feather of the same weight as a steel ball will fall slower unless you are in a vacuum.

So what is the effect? Normal bodies have a parabolic trajectory when thrown (or jumping), your ninja would however be able to change his parabolic trajectory more upward or downward mid flight. With practice and if you can change mass really fast you can reduce the effect of air drag while moving or stop really fast by decreasing your mass to nearly zero.

If you could change the density of parts of your body you could play a lot with momentum. Think about the ice skater whirling around with arms spread. His speed will increase when he pulls in his arms due to the decrease in momentum. You can do the same by reducing the density of your arms, giving you a whole range of options with regards to flexibility and agility.

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  • $\begingroup$ Pardon my late reply but this is very interesting and very well might be the answer I was looking for. Thank you ! $\endgroup$ – Anonymousworldbuilding Apr 11 '20 at 19:36
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Mostly no

The ability to lower one's mass is very useful, especially in an atmosphere. A 70 kg person who can fall from a great height and survive by lowering their mass to 5 kg while retaining the same volume will have an easily survivable terminal velocity. Such a person would also be extremely popular in the space program, as they could be launched into orbit relatively cheaply. They would also be good at one archetypically ninja trick in that they could walk across a nightingale floor without creating noise, due to the miniscule force they exert on the floor. They could creep across structurally weak roofs without falling through, climb up and down on incredibly light and weak lines that would not support a normal human, get rescued from the slopes of a volcano by a normal-sized eagle etc

However, much depends on whether muscles, tendons, bones etc with a reduced density are as strong as when the same tissues are at normal density.

  1. If tissue strength is proportional to density then the would-be ninja is no better off than a normal person in most situations. For example, if they jump upwards while at normal density and strength and then lower their density, their maximum jump height will actually be almost completely unaltered - the same mass providing inertia will be acted on by gravity proportionally.
  2. If tissue strength is unchanged regardless of density then this is basically a super-strong individual, although limits on human muscle speed will mean that not all activities will be created equal. For example, maximum running speed is largely limited by muscular contraction speed, even when running downhill there are limits to how quickly a person can run. Punching / kicking are in a similar situation, so there is no real advantage to decreasing mass while striking and then going to "normal" a fraction of a second before impact. High jump would be improved but not totally ridiculous for the same reason. The "ninja" would do best to give up on trying to be unseen and instead compete in the Tour de France - a trained cyclist who is only needing to propel a 5 kg body and lightweight bicycle would have an overwhelming advantage in the uphill stages and can resume their normal mass for the downhill runs.

Finally, flexibility is dependent on how far muscles, ligaments and tendons will stretch. There is no reason that magically reducing the mass of these body parts would change their elasticity. Any potential mechanism for increasing elasticity due to loss of mass would imply a loss of strength, which would offset other advantages. There is no reason why a character could not train to gain flexibility through ballet, martial arts, yoga or other means, but this would be unrelated to a magical ability to break the laws of inertia.

This issue has been explored in one or more fantasy books which I have completely forgotten the name of or I would give credit. The protagonist was able to both decrease and increase his weight or mass, for example, he was able to fall lightly from a height or pin an opponent to the ground.

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  • $\begingroup$ Thank you for the insight ! Your answer is thorough and easy to understand. I guess I'll just chalk the super-humans feats to magic then. $\endgroup$ – Anonymousworldbuilding Apr 11 '20 at 10:56
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    $\begingroup$ It seems to me that in point one you ignore friction. In a vacuum you are of course right, but the ability to decrease density would also alter the ratio of volume (mass) over area. $\endgroup$ – D.J. Klomp Apr 11 '20 at 11:09
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    $\begingroup$ @D.J.Klomp correct, but the effect of air resistance on a "normal" human jump is negligible. If density was reduced massively then jump height / length would decrease due to the increased effect of air resistance - started to draft the answer that way but it was getting too lengthy and the main point was simply that it didn't help, not that in extremes it made jump performance worse. $\endgroup$ – KerrAvon2055 Apr 11 '20 at 13:38

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