The System in General

So, the magic system involves mages being able to create semi-tangible, rope-like energy structures. I'm not gonna go into too much detail on it since this isn't the point of this question, but basically imagine it as a mix of Just Cause's tether mechanics, the Pull/Push powers from Mistborn (Minus the push), and Spiderman's webs, but made out of energy.

So, the system has an overall theme of kinetic/mechanical energy. For the sake of thematic cohesion— and in order to have a magical source that sooort of follows the first law of thermodynamics without resorting to to "they eat magically charged salt"— I came up with this idea: What if base energy for this magic system is gathered through the users absorbing nearby kinetic energy?

Now, this could work a variety of ways and have interesting effects in the environment around them. BUT, in order for them to not instantly have a secondary power that allows them to essentially freeze time (or at least motion) in a small area around them, I've decided to set a few limitations.

Levels of Kinetic Absorption

A "Weaver" (aka a magic user of this system) can absorb instances of kinetic power from around them in power proportional to their skill. So for example:

  • a Weaver that is just starting would be able to absorb sound vibrations from the air around them, creating a sphere of absolute silence.
  • A more advanced user could, in addition to sound, absorb more subtle movements, making swaying grass freeze around them, or falling raindrops to stop mid air or slow down in a sphere around them.
  • Really advanced users could create a "Dampening field" around them that would make others feel as if they were moving through honey instead of air. In order for that to not be so overpowered, like for example making people's hearts or blood slow down and depriving them of oxygen, I'm gonna say this "absorption field" works in a projection-like way and can't pass through solid surfaces.

Extreme uses of this could allow users to walk on water, or even near-fully dampening kinetic power from around them, freezing things mid-air.

Now, I know this is basically a whole other magic system, but the more extreme examples will be kept for characters that have a legend-like status. The sound and slight dampening of small motions would be much more common.

How Effective would it be?

Now, my main question here is how long would it take to stockpile a significant amount of energy through those methods? For example, how long would it take for someone sitting down a cliff near the ocean, absorbing sound from the environment, to collect enough energy to fling themselves up it?

Would this provide too much, or too little relative power overall? And what other sources of kinetic energy would there be in an outdoors setting?

Important note: This story would take place in an Age of Sail sort of setting, on a world that's for the most part a massive archipelago.

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    $\begingroup$ So can someone fling themselves off the mentioned cliff, then absorb the kinetic energy continuously over hours/days of NOT falling (until they actually fall, they have kinetic energy constantly generated due to gravity) or is there some limit in the total potential energy contained in something that you somehow harvest? There are some weird paradoxical possibilities. Don't forget that the ultimate default form of kinetic energy is thermal - can they absorb heat from the air to power spells? I would go with potential (useable) energy, not total kinetic energy, so stopping winds, for example. $\endgroup$
    – DWKraus
    Commented Aug 22, 2021 at 2:43
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    $\begingroup$ I might advise that sentient life has an innate magic field that prevents caster from directly affecting their bodies - so you could thicken air, but not stop hearts. You might still be able to suffocate someone by preventing the movement of air into the area of their head, though. $\endgroup$
    – DWKraus
    Commented Aug 22, 2021 at 2:46
  • $\begingroup$ So, to answer those questions in order: In order to absorb the motion of themselves falling a user would need to be quite advanced. They couldn't fully stop themselves from falling, only dampen the motion. I may make it so they absorb the acceleration of an object, so that even if they're sapping away it's speed it is still falling at a constant rate due to gravity. As for thermal, I actually have another system that works like that, so. Again, gonna keep it to sound/acceleration. Finally, I do like the idea of some sort of mechanism that protects living beings from extensive damage. $\endgroup$
    – Magbread
    Commented Aug 22, 2021 at 11:37
  • $\begingroup$ As I said in the answer bellow, maybe a cap on the weight of things someone can "freeze" and harvest the gravity from would be effective. That way someone can't just passively float -and- collect the energy from their own weight being pulled by gravity. $\endgroup$
    – Magbread
    Commented Aug 22, 2021 at 11:51

1 Answer 1


One example:

Say we count ambient temperature as kinetic energy. And say someone wants to lift 100 kg by 100 m. Acceleration due to gravity is about 10m/s^2. Multiplying to get the desired gravitational potential energy, they need to absorb 100 kJ.

The specific heat capacity of air is about 1 kJ / kg K (https://www.ohio.edu/mechanical/thermo/property_tables/air/air_Cp_Cv.html).

So if they can lift the weight if they lower the temperature of 10kg of air by 10 degrees. The density of air is around 1kg / m^3 (https://www.macinstruments.com/blog/what-is-the-density-of-air-at-stp/), so this is about 10 m^3, or a sphere of 1.3m.

But maybe that's too overpowered, so let's consider sound. Sound travels, so a sphere of silence will be absorbing all of the energy entering the surface of the sphere. I found this article: https://engineering.mit.edu/engage/ask-an-engineer/can-sound-be-converted-to-useful-energy/

It says very loud sounds might be 0.01 w/m^s. Say they have a sphere of radius 10. The surface area is 4 pi r^2, so about 1000 m^2. If loud sound is hitting the sphere from all sides, that's about 10 watts. 1 watt is 1 joule / second, so it will take about 10,000 seconds to absorb 100 kJ (almost 3 hours) if they're surrounded by jackhammers. But ambient sounds are probably much quieter. Decibels are a log scale: every 10-decibel decrease in volume is a 10-times decrease in power. So realistically it will take many orders of magnitude more time to absorb the energy. Go down from 120 decibels to 70 decibels, and it will take 10000 times longer to absorb the energy - going from hours to decades.

What about absorbing the kinetic energy of moving objects? Kinetic energy is 1/2 * mass * velocity^2. I'd guess grass might wave around 0.01 - 0.1 m/s (a very rough guess), so in one instant that might be 0.00005 - 0.005 J/kg of grass. The question is how often this is refreshed - does the grass constantly absorb all of the energy from the wind pushing against it, or does the person have to wait for it to sway to absorb the energy? If they're absorbing the energy from 100kg of grass twice per second, that's maybe 1 watt, and they need 100,000 seconds (about a day) to absorb the energy. If the grass is acting like windmills, they can probably absorb more energy.

If they can directly take energy from ocean waves or large volumes of wind or large trees blowing in the wind or seismic waves reaching the surface, they could potentially absorb huge amounts of energy very easily.

  • $\begingroup$ Thanks for the analysis, it's really helpful! So, as far as I understand sound would not be a very effective source of energy, especially since this is a pre-industrial setting. No sound would most likely be a side-effect of absorption then. Now, I think things would work similar to windmills yes, so if something was applying force to a "frozen" object, that force would be absorbed to the user's stockpile instead. In order to not have someone say, infinitely stockpile energy from gravity by freezing a boulder, I'll say that the more mass something has the harder it makes it to freeze. $\endgroup$
    – Magbread
    Commented Aug 22, 2021 at 11:44
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    $\begingroup$ So for example, a novice could gather energy by freezing a coin in the air above their hand, while a master could do the same with a heavy weapon or weight. Now, I'm not fully sure about this since it seems a bit too easy to "farm" energy, it does make an interesting visual though. Maybe a stricter cap on the weight of things they can freeze would force them to absorb from many smaller sources of energy rather than one large one? $\endgroup$
    – Magbread
    Commented Aug 22, 2021 at 11:48
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    $\begingroup$ You could think of it like a waterwheel. There has to still be some movement to get energy out, and there's a maximum amount of force it can support without breaking. It slows movement but doesn't extract more gravitational potential energy than is present. $\endgroup$ Commented Aug 22, 2021 at 12:10
  • $\begingroup$ Yeah something like that. When extracting energy even the most powerful users can't completely freeze things, just slow them down/dampen their speed significantly. $\endgroup$
    – Magbread
    Commented Aug 22, 2021 at 12:41

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