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Imagine a world of magic. This magic can do anything you want it to, as long as physics agrees (totally useless addendum: physics is actually a very pendantic guy, but rumour has it his mistresses have been seen flying).

Meaning, energy is to be conserved. A spell can lay dormant for centuries, but once activated it can only consume the amount of energy put into it when casted. Which is why magicians are very good cooks, they have to get fat fast before casting a powerful spell and then consume their fat all at once to put the energy into the spell (others just kill animals, but that's frowned upon).

Now skeletons are easy to use as tomb guards (scary, bones are easy to get), but how much energy would be needed (approx.) to have a skeleton actually be able to fight? They don't have muscles etc. so keeping all those bones aligned would be consuming some energy already. Is it feasible to cast the skeleton spell and the remains of an army within a few days/weeks/months without having big energy sources available (aka food, maybe a companion), or would those skeletons rather need more energy?

"Careful, there are skeletons there, they'll chop you into parts!" "I know, just watch." The hero charges into the dungeon activating all trap spells, turns around, sprints out of the dungeon, releases the stick holding the boulder next to the door, waits two days, removes the boulder and walks into the dungeon because the skeletons had only enough energy to hit the boulder once or twice and are now just bones lying on the ground.

I'd like to evade that scenario.

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    $\begingroup$ You mentioned energy to keep them together, not time or money. That really depends on your economy and materials used. Soft plentiful metals are easy to carve runes in adn shouldn't be to expensive. Rare hard metals will be hard to work with and take considerable time and knowledge. Perhaps someone will sell DIY kits. Mastersmith and runemaster or something. $\endgroup$ – Mormacil Mar 20 '17 at 13:53
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    $\begingroup$ Skeletons seem a waste of energy beyond their initial scare value. To make a curse real, re-animate nasty viruses and bacteria. Or poison the tomb for even less energy. $\endgroup$ – user535733 Mar 20 '17 at 13:54
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    $\begingroup$ Reminds me of a story from The Elder Scrolls. Unknowingly people take the bones from undead vampires to make armor out off. But merely sleeping and being undead led to those vampires waking up with no bones. So they're this blanket of flesh and muscle looking for revenge. $\endgroup$ – Mormacil Mar 20 '17 at 13:58
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    $\begingroup$ You mentioned animals, is there a way to power a spell by having it "eat" a living animal? Drop the sacrificial lamb into the magic circle, it shrivels to dust and the magic is powered for another few years? $\endgroup$ – Murphy Mar 21 '17 at 12:07
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    $\begingroup$ How long you need the skeletons to last would be a good thing to add. $\endgroup$ – John Mar 21 '17 at 14:04
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I mean, looking at it purely at a caloric perspective, a skeleton by itself weighs about 30 lbs. That's not much!

Fencing, at 150 lbs: 288 kcal/hr -> skeleton: roughly 57.6 kcal/hr

Karate, at 150 lbs: 720 kcal/hr -> skeleton: roughly 144 kcal/hr

Wrestling, at 125 lbs: 360 kcal/hr -> skeleton: roughly 86.5 kcal/hr

So depending on how the skeleton fights, it burns anywhere from 50-150 (roughly) kcal/hr. A Big Mac is 563 kcal, so a big mac's worth of energy powers your skeleton fighting as a fencer for 11 hours, as a wrestler for 6.5 hours, or doing karate for about 4 hours. Assuming standing there not moving doesn't use any energy, for a few Big Macs you can get quite the charge out of your fighting skeletons.

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    $\begingroup$ Uh, the values are not correct. A human has only 30% efficiency, so you cannot use human values for comparison. A skeleton does not need maintain body temperature, vital functions or muscle contractions. $\endgroup$ – Thorsten S. Mar 20 '17 at 14:50
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    $\begingroup$ The question says that "keeping the bones together would be using some energy already", so I figured that upkeep was roughly equivalent to "human values" for comparison's sake. Granted, the pure weight conversion is probably imperfect, but I feel like it's the best way we can simulate a magically animated skeleton. If anything, the skeletons could probably be more efficient than my numbers above, meaning Big Macs are that much better at fueling your skeleton dungeon. $\endgroup$ – SpaceMouse Mar 20 '17 at 14:59
  • $\begingroup$ Remember that a skeleton will have far less mass moving a weapon, so has to compensate by moving it faster. $\endgroup$ – Sherwood Botsford Mar 21 '17 at 22:21
  • $\begingroup$ This was really the answer I was looking for, the exact numbers are not so relevant, but that means a fat mage could actually raise an army. (Around 10-100 skeletonhrs per kg fat) $\endgroup$ – DonQuiKong Mar 22 '17 at 11:31
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Surprisingly little.

1 kg fat tissue amounts to an energy of 7000 kcal. That means 1 kg fat is able to approximately boil 70 kg ice water. Even better is the mechanical equivalent: 1 kg mass can be elevated 427 m for one kcal, so 1 g fat can lift 1 kg 3000 m. A humanoid skeleton weighs only 12% of the body, so you can multiply the amount of energy with the factor 8 if you use normal mass.

The reason humans need energy is that our muscles consists of fibers which must fire involuntarily. This allows fast movement, but it is instead possible that molecules are simply blocking. The skeletons can freeze and consume no energy at all.

Now ferocious blows and movement.
Really ugly is it if the skeletons can store superfluous kinetic energy. Then only rising up will be a problem (and they get the energy back when it goes down). More realistically you can assume that going up needs all energy and going down will only consume 10% (While they go down, they need to stop).

1 gram fat tissue amounts to moving 1 kg approximately 3 000 m. That's a lot.

A ferocious blow will be e.g. a 5 kg warhammer coming down with a speed of 30 m/s. That's 2250 J = 537 cal. So 1 g fat = 7000 cal is equivalent to 15 heavy blows with a warhammer.

If energy is conserved, your skeletons are a real pain in the ass.

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As per a clarification, sacrifice of living creatures can fuel magic.

It's "not nice, but possible".

So it's possible to keep a really long term magic spell fueled with "this one cool trick, tomb raiders hate him"

It's socially frowned upon to sacrifice animals. How can we minimize the "euck" factor?

ANTS!

Ants

You want the spells that defend your tomb to last not just a century but for millennia, you don't want tomb robbers to be able to beat your protections simply by running down the batteries on your magic warriors.

So you need some kind of steady trickle of power.

Bring on the ants. In some ancient structures you'll sometimes get a hole in the roof and insects falling down and being unable to get back out.

You construct your tomb to do this purposely. A variety of little chutes and gratings allow insects to fall down from the soil above the tomb, they get funneled towards a sacrificial circle where they are consumed to fuel the magic.

Depending on taste there may even be a few skeletal guardians tasked with keeping the magic circles clean and clear.

While tomb raiders can reduce the magical reserves of your defenses this small power source will ensure that they can't completely deplete them.

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The amount of energy would vary dramatically based on the style of fighting used. Many martial arts emphasize efficiency over brute force power and prefer to use the enemy's force against them. Aikido and Judo are famous examples of this sort of fighting. Skeletons which use these styles would require far less energy than those trained in, say, Muay Tai kickboxing, which is so energetic that it's spawned off exercise routines loosely based on it!

In general, grappling arts have an advantage in your magical realm because grappling technically doesn't call for energy. While you may be using great force on the other person, force itself doesn't require energy. Energy is force times distance, and you often don't move very far in a grapple. Real human beings must expend energy grappling because of the inefficiencies built into how our muscles operate. Your skeletons would not have such a disadvantage, so they could grapple much longer with less power. Of course, they also may weigh less, which is a substantial disadvantage in grappling, but that might be artificially resolved with weights tied to the bones (at the cost of requiring more energy to move)

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Magic and physics don't mix together well. How much energy does it take to animate a skeleton? Well, how much do you want it to take? It's magic, after all.

A skeleton is bound together by organical tissues, like muscles and sinews. Without them, a femur it's not connected to the hip or knee by anything, so your animated skeleton are nothing but flying bones. How much energy does it takes to maintain them flying? By which means (antigravity, electromagnetical fields)? Why using skeletons at all? Why don't animate a knight's armor (a là Full Metal Alchemist)?

I mean, I don't want to downvote, but the amount of handwaving this situation requires makes the point about energy uses and conservation a little moot.

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    $\begingroup$ The handwaving to some extent is part of the story. It's meant to be funny and stupid but should be conclusive in itself. So mages using skeletons is a tradition which is totally not useful, but as the moving of the skeleton requieres energy, that energy has to come from somewhere. Sinews don't need energy to just keep two bones connected so the magic doesn't need energy for that unless some force is applied to the bones (e.g. smb is trying to oull it apart), then the energy consumed (okay, this part is not 100% physics based) is the energy which would be needed to pull sth with that force. $\endgroup$ – DonQuiKong Mar 20 '17 at 15:08
  • $\begingroup$ And with no energy I mean almost no energy but that didn't fit into the previous comment. E.g. gravitation has to be constantly counteracted even to just keep the bones levitating. $\endgroup$ – DonQuiKong Mar 20 '17 at 15:10
  • $\begingroup$ Then I go with the rest of comments: very little indeed, given its low mass and no other requirements of energy (they're dead, after all). $\endgroup$ – Rekesoft Mar 20 '17 at 16:08
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Skeletons aren't animated by magic levitating/actuating joints directly! if that were the case, surely it would be much simpler and more effective to animate the sword/spear directly!

Skeletons work because they in some way maintain the idea or concept of a humanoid body, even without muscles, ligaments, sensory organs, or brain. This is also why skeletons can usually make some sort of rudiementary noises, despite, obviously, not having vocal cords or lungs, and why shattering their bones, even structurally non-essential ones is the main way to destroy them.

Going out on a limb, the withering away of the rest of the body to the bone is symbolic of the stripping of the mind down to aggressions, fighting instinct, and tactical judgement.

Now, as to the cost - since the effect is purely symbiological magic, it is impossible to quantify given the absence of observable magic in our world. The best we can do is compare the effect to other commonly discussed magical effects. Given that the essence of the spell is "to preserve a distilled idea of something from its unrefined form" animating a skeleton would be similar in cost to, for example, making a sword magically sharp or armour hard to break... so, small fry stuff, unless the scale of the effect is very strong, long-lasting, or particularly resistant to disruptions to the physical component

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Create a double spells. One to wake up and give energy the skeletons ( I assume they are sentient on their own or by other necro enchantment). And second to take energy from visitor. Anyone who enter is drained from energy stored in his body. Remember the super aging guy from Indiana Jones? He was actually under the "drain energy" spell.

Advantages

  1. Your skeletons are animated only for the time needed, so as long as anything with energy is in the tomb
  2. If you are visited by a pack of adventurers the leanest, smallest of them die first without even facing your guardians. Now that's what I call psychological effect.
  3. You can use the bodies as your new warriors so the energy needed is increasing with time and it's harder and harder to get further in the tomb because the amount of skeletons rise and adventurer energy decline with every moment.
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  • $\begingroup$ Nice idea, but with the way my magic works a counter spell would be easy and the energy-draining spell would have to have more energy than the counter-spell. if the counter-spell feeds directly of the visitor, the visitor would lose as much (~+- some conversions etc.) energy as is stored in the energy draining spell and then the energy draining spell would be gone and no skeleton attacking them. $\endgroup$ – DonQuiKong Mar 21 '17 at 8:44
  • $\begingroup$ But you would need to know that what counter-spell to cast. Anti aging or anti energy drain. Also you could have some energy storage so any astray animal would stack his energy. So a drain energy spell at the entrance (let's call it lvl.1) and skeletons on lvl. 2. So there you would need to fight with skeletons but also cast anti-spells on yourself. Maybe a "drain energy" from spells themselves? $\endgroup$ – SZCZERZO KŁY Mar 21 '17 at 8:59
  • $\begingroup$ Counter spells can be stacked, cast on a medaillon, etc. Draining astray animals would work. I'm not sure I would want dungeon entrances stacked with dead animals though. But it's an interesting idea. Maybe skeletons start draining energy from killed/wounded opponents by eating them and then converting their bones into new skeletons. Would for sure clean the dungeon up a little. $\endgroup$ – DonQuiKong Mar 21 '17 at 9:07
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    $\begingroup$ Maybe modify the spell so the "animate corpse" would work only for humanoid ones and the drainage would work even on bones (as bones can be burned which produce energy) of animals so you would have clean dungeons (well with maybe a ash here and there) $\endgroup$ – SZCZERZO KŁY Mar 21 '17 at 9:21
  • $\begingroup$ Self multiplying and self supplying skeleton armies, I like that. $\endgroup$ – DonQuiKong Mar 21 '17 at 9:44
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You're overworking this. You use a soap bubble spell. This creates a pressure equalizing, but gas impermeable force field near the mouth of the cave.(The boundary would shift back and forth with changes in air pressure. Inside that you have no oxygen. People can walk through a soap bubble spell, but lose consciousness a few steps later.

Minimal energy required.


In general it's a good idea for Magic world to have some form of laws or rules. Doesn't have to be conservation of energy/momentum, but there is no reason why not. One of my disappointments with the Harry Potter series is the lack of cost for magic.

Niven used Conservation of Mana for one series of stories.

Conservation of gravitational potential energy would allow you to float anything at a fixed height (above sea level) at no energy. This makes mountain ports valuable since they could have docks at various heights.

You could have lots of fun with teleport spells, and people figuring out conservation of energy and momentum, as they arrive at a distant location with a velocity of 100 kph straight down.

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I figure imbuing a skeleton with the energy to work for years if not decades or longer would take quite the investment. I don't think more then one or two a day could be made, probably less.

You're transferring energy to create a thing that moves, thinks and anticipates. It must be able to fight effectively. Not a trivial task.

Perhaps they can be created with a minimal lifespan and then recharged at a later place? That would speed up creation as recharging could be given to another person. Maybe sucking dry the corpse of a prisoner, removing all life force. That would make being a wizard prisoner of war a very bad thing to be.

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  • $\begingroup$ Unless fighting they don't do anything and I figure most fights are over pretty fast, so the energy worth a few hours of fight would suffice I think. Thinking .. Yeah no, more like hitting stupidly at anything that moves. $\endgroup$ – DonQuiKong Mar 20 '17 at 14:15
  • $\begingroup$ That would allow exploitation like moving a shield in front in shifts till the skeleton is depleted. You need some intelligence. $\endgroup$ – Mormacil Mar 20 '17 at 14:25
  • $\begingroup$ that's an okay way to defeat the skeletons, but you hold that shield when twnetythousand of those beasts are charging at you - otherwise, the skeletons are really not meant to be overpowered or even strong, it's more of a tradition to summon skeletons of the foes you have beaten (or who's bodies you found, that's nitpicking) to defend your corpse so nobody does that to you. Among other measures. $\endgroup$ – DonQuiKong Mar 20 '17 at 14:34

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