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I see that similar questions have been asked before, but I have yet to find a sufficient answer. I'll try to keep it short. I'm currently working on a story involving a soldier who discovered he's capable of manipulating kinetic energy. As he advances through the story, he learns more about what he can and cannot do with his new abilities. For example, he's capable of absorbing the kinetic energy fired from bullets or thrown from punches before storing it in his body. This pool of absorbed kinetic energy can then be imparted into other objects to "charge" them.

Outside of his hand-waved ability to store and impart kinetic energy, I'm trying to keep the actual applications of his abilities grounded in reality, at least somewhat. What are some applications an archetypal soldier would use this power for?

Here's one example I haven't fully figured out yet:

-If this character were to be shot directly by a tanks main gun, he could absorb the shells energy (assuming its solid-shot AP or sabot) and walk away unscathed. The shell would then harmlessly fall to the ground, deprived of all of its speed. If he were to then take that same energy and charge a 7.62mm bullet with it, how would that bullet respond to the increase in energy? Would it shoot faster? Weigh more? Immediately melt from extreme amounts of heat? If shot back at the tank, would it penetrate or just shatter? Does this question make no sense because he wouldn't be charging it with kinetic energy, but something else?

Edit: It's clear now that he wouldn't be "charging" the bullet with kinetic energy, but something entirely fictitious. If, due to this fictional energy, the 7.62mm round had somehow added the force of the tank shell to its net force, would it penetrate the tank or just fragment on the armor?

Any input would be greatly appreciated.

-----Edit 12/29-----

Thanks for the quick replies guys. After reading some of the comments and cross-checking with my own research, obviously my character would need more hand-waving than I had originally thought. Much of what he could and could not do would lie with me, the writer.

-----Edit 12/30-----

I forgot to mention a few of the specifics regarding his ability when initially writing this. My mistake.

Some things to consider:

-I'd like to avoid any heat-related powers at the moment.

-The character can only "absorb" what he comes into contact with -- he cannot drain energy remotely.

-The capacity of his storage is not limitless. At the uppermost limits, he could absorb the equivalent of maybe one-half to one kiloton worth of explosives before dying from an overcharge. This is an arbitrary value I slapped on for the moment, however.

-He needs to be awake and conscious for his ability to work. He's capable of continuously absorbing energy during combat to defend against bullets and explosions but it is an entirely conscious effort to manage his defense. If he becomes too weary or distracted it's entirely possible a bullet could sneak through.

-He doesn't need to time a bullets or other objects impact just right for it to work, he just needs to have it active at the time it does impact.

-His stored energy bleeds out over time if it isn't used.

Thanks again for the feedback and thank you for bearing with me.

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    $\begingroup$ I would go with this not making any sense. Kinetic energy, generally, needs direction - otherwise the kinetic energy logically (I am making some assumptions here) would have to be imparted to the individual molecules (already in random agitation) which would be the same as heating the bullet - yes it could melt, but would probably explode first when the percussion cap ignites. I can't see this being very workable. $\endgroup$ – Lee Leon Dec 29 '17 at 11:47
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    $\begingroup$ I think perhaps you need to work out where your super-ability starts and where reality ends, and vice versa. For example, at what point can he take that kinetic energy, does the shell need to touch him first? Logically, the process of taking the energy should be similar to, and the reverse of, imparting it. $\endgroup$ – Lee Leon Dec 29 '17 at 12:07
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    $\begingroup$ "If this character were to be shot directly by a tanks main gun, he could absorb the shells energy and walk away unscathed. The shell would then harmlessly fall to the ground, deprived of all of its speed": please do tell, where does the momentum go? And, you do understand that "shells" contain high explosives, of which the energy is quite a bit larger than the kinetic energy of the shell? $\endgroup$ – AlexP Dec 29 '17 at 14:18
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    $\begingroup$ Then answer is entirely up to you in deciding how your magic works. $\endgroup$ – John Dec 29 '17 at 15:04
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    $\begingroup$ You've changed the question to the point where your soldier's ability can be expressed in any way someone could use an infinitely hard baseball bat. It does not change the mass or composition of the affected object. Indeed, it's as likely to destroy the object (e.g. a bullet) as it is the target the object is redirected to. $\endgroup$ – JBH Jan 3 '18 at 3:48
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Scales

How much energy would your character be able to pull from items? Here are a few options:

  • Pistol : a few hundred Joules
  • Rifle : a few thousand Joules
  • Heavy weapons : a few tens of thousands of Joules

Rate of Energy Transfer

Power (unit of Watts) is the engineering term for the amount of energy (unit of Joules) per second transferred between two objects. Does your hero have a maximum amount of energy per second he or she can absorb?

You can use the scale above to help figure out what your hero's "strength" in terms of how much energy he/she can absorb or push per second.

Let's say your hero can absorb the energy of a rifle round, rendering it harmless. Here is how close he can get to the muzzle:

  • Touching the muzzle (stop the bullet in ~ 1mm) : 1 billion watts (1 GW)
  • Close range (1m / 3ft) : 1 million watts (1 MW)
  • Moderate range (10m / 30ft) : 100 thousand watts (100 kW)

The equation involved is $\text{Power} = \text{Energy Absorbed} \div \frac{\text{range}}{\text{muzzle velocity}}$

Range

At what range does absorbing or adding kinetic energy work? Must your character be in physical contact with whatever is adding energy to him/her, or whatever he/she is adding energy to? Or is it a ranged effect?

Not Totally Stopping Bullets

There's an inverse square relationship with cutting energy to cutting a bullets remaining velocity. As an example, cutting a rifle round's energy by half, only cuts the velocity be the square root of 2 (~1.4 x)

Past a few multiples of the speed of sound (~343 m/s) light armor is pretty ineffective. Your ex-soldier character might still wear body armor to deal with the occasion when a bullet can not be caught.

In the low-speed armor-is-effective regime, penetration scales approximately linearly with energy. So, if your hero can cut a round's energy by half or two thirds, he or she is also doubling or tripling the effective thickness of his/her armor.

Here is the penetration equation: $$\text{P} = \frac{1}{2} \times {\text{B}_{\text{v}}}^{2} \div ((\frac{\text{B}_{\text{a}}}{\text{B}_{\text{m}}} \times \text{A}_{\text{ys}}) + (\frac{\text{B}_{\text{a}}}{\text{B}_{\text{m}}} \times \text{A}_{\text{d}} \times {\text{B}_{\text{v}}}^{2}$$

Where:

  • $\text{P}$ = Penetration.
  • $\text{B}_{\text{v}}$ = Bullet velocity.
  • $\text{B}_{\text{a}}$ = Bullet area.
  • $\text{B}_{\text{m}}$ = Bullet mass.
  • $\text{A}_{\text{ys}}$ = Armor material yield strenght
  • $\text{A}_{\text{d}}$ = Armor material density.

Adding Energy to Munitions

The inverse of absorbing the energy of projectiles. The hero would need to be careful not to add too much energy (past a few hundred meters per second, energy is much less relevant than the length and density of the bullet - this is why .50 cal rounds can be stopped by a few feet of water, per Mythbusters).

As long as too much energy isn't added, your hero could double or triple the effectiveness of the weapon against light armor, and likewise increase or decrease effective range.

If your hero has a certain amount of control, he or she could impart that kinetic energy to spin certain weapons that don't have the benefit of muzzle rifling (rocks, javelins, spears) to give them gyroscopic stability and improve accuracy.

Throwing Things Around

An obvious use of this ability is to throw people and things around. It would take 1,000 Joules to throw an average weight person around at 10 m/s (1 kW)

Hold Someone or Something in Place

Kinetic energy is movement. So, opposite of throwing things around, your character could hold something firmly in place : a locked door, a wall, an opponent, or game when hunting.

Cutting Things

If your character can shape the kinetic energy he/she imparts, he or she could split logs with a few kilowatts of power. With a few megawatts of power, he/she could split steel.

Shadow Boxing

Similarly, the hero could send the kinetic energy from his moves somewhere else (splitting logs remotely by going through the motions on his porch), or relocating a punch/kick downrange.

Surgery

If the hero can use his or her ability at range, he could use it to help remove bullets, set bones, clear blockages (or create them) without incisions.

Never Hurt by a Fall

You character could convert his absorb his own kinetic energy and apply it to slowing his or her velocity. As a result, your character could fall from any height without injury.

Wattage "strength" required: ~500 Watts for an average adult (100kg)

Walk on Water / Air

Likewise, if your hero can cancel his downward kinetic energy and keep the horizontal kinetic energy, he can walk from rooftop-to-rooftop, or walk on water.

Wattage "strength" required: ~500 Watts for an average adult (100kg)

Similarly, the hero could make a paper airplane, golf ball, arrow or thrown rock have an range limited only by air friction.

Flight

Depending on how long your hero can store energy, he or she could redirect excess into himself for some amount of flight. When out of the stored energy, he or she could glide down by absorbing and canceling his/her falling kinetic energy.

Super Cold / Super Hot

Up until now, I've assumed your hero could only influence one thing at a time. However, if your character can absorb kinetic energy from everything in a specific volume of space, your character could effectively control temperature (temperature is mean kinetic energy of molecules).

At only a few hundred kilowatts, you could freeze air a cubic meter of air from room temperature to absolute zero.

The opposite is also true. Bear in mind that a nuclear weapon has a thermal energy of only a few trillion Joules (tera Joules).

This could be useful in stopping engines (jet, rocket, internal combustion) cold. Actually, for engines with moving parts, your hero could still cause the engine to seize.

Alternatively, this could be used to neutralize minefields by cooking off the explosives, or cooking off the ammunition that bad guys are carrying.

Making Laser Weapons More Effective

There is a property of the atmosphere called blooming that limit the effectiveness of laser weapons. What this is is the atmosphere heating sufficiently under the laser energy that the atmosphere's optical characteristics change, dispersing the energy. If the hero can cool things down, he or she could make use of modern laser weapons much more effectively.

Detonating Things

If you can superheat things, your character could cause them to explode. The overpressure is the equal to the energy provided to the material divided by the volume of the material. Something like a steel casing would require a few giga Pascals of pressure to rupture.

Creating Light and Darkness

60 Watts of power is sufficient to light a light bulb. Similarly, if the ability works at range, the hero could create darkness by absorbing the energy from the lights. Wouldn't work with moonlight or sunlight because the origin is extremely far away.

Creating Silence or Noise (or Ventriloquism)

Sound is kinetic energy in air. So, it would be possible for your character to silence an area. Or to create a noise (like a flash-bang to disorient opponents). Or, with fine control, throw his/her voice, or create understandable sounds (music) at will.

Getting Crazy

If your hero can stop a rifle bullet at touch range, he or she has enough wattage "strength" to start talking about bending space-time (generally energies in the tera Joule range).

The equivalence principle proposes that acceleration and gravity (both buildups of kinetic energy) are effectively the same. Further, that energy bends space. This opens up a lot of venues not always taken advantage of in fiction.

You could bend light (and laser energy) away from a point (only takes a peta Joules, or one thousand tera Joules). Not quite black holes (that would take a few dozen hepta Joules), but you could make small changes. You could also slow down time.

The kinetic energy required depends on the magnitude of the effect and the radius effected: $\text{Energy required} = 10^{43} \text{Joules} * (1 - \text{gamma}) * \text{radius}$

Maybe not.

Keeping it Under Control

It might seem like this character is overpowered, but maybe not.

If he shoots himself, he gets a quick bit of juice. Not too much. If he or she sticks his fingers in an outlet to absorb the kinetic energy from frying him/herself, that's still about 3.3kWatts (not too much).

If someone punches or stabs him, a few hundred watts.

In most cases, the hero is stopped because there's just not that much energy being directed at him. If you do allow the hero to absorb and send thermal kinetic energy, you might allow the hero only to absorb thermal kinetic energy above standard temperature and pressure, which is trickle (80 Watts for a candle).

On a normal basis, the only thing the hero could do proactively (without reacting to some energy) is walking across roof tops, on water, never getting hurt on a fall.

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  • $\begingroup$ Impresive! You cover every aspect of his ability, even aspect he would never know! $\endgroup$ – Ender Look Dec 29 '17 at 23:42
  • $\begingroup$ Exceptional answer. You seem to have understood the question quite well. I forgot to mention his limitations in the original post, but I recently added a few thought's to consider in a recent edit. He definitely has his limits. Thanks for the input! $\endgroup$ – DMQ Dec 30 '17 at 6:46
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If we're being completely logical, this soldier would be much more useful off the battlefield where he can turn any sort of motion into electrical energy by turning a turbine - wind, waves, enemy airplanes overhead, anything that moves gets turned into free electricity. Or he could use the energy to transport things. He could probably get things to orbit much more cheaply than is currently possible - just store enough energy to get up there and then ride the rocket all the way up. He doesn't even need a return capsule, he can just jump off the rocket and manipulate his own kinetic energy to fall back to Earth nice and slowly. Who cares about radiation? He can just recharge his powers by absorbing the energy of the charged particles around him.

But seeing as you want this to be an interesting story, and not one about battlefield logistics, he could make an incredibly good air-defense system. Depending on the range of this ability he could literally stop aircraft and missiles in their tracks, letting them fall harmlessly wherever he wishes.

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  • $\begingroup$ This is in the ballpark of what he would be capable of doing, but his range is more or less limited to what he could touch. Maybe not at the start of the story, but maybe later on he could absorb energy from nearby targets using the air as a conduit, though with diminishing returns. It's more of a "mid-scale" power as opposed to a godlike one. $\endgroup$ – DMQ Dec 29 '17 at 21:25
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I figure this will fall somewhere between Comment and Answer, but this could work.

Have his body take the Kinetic Energy/ Momentum/ Whatever/ and store it as potential energy in his handwavium glands like a stretched rubber band, or whatever mechanism you choose. This happens when he gets impacted by more than X Joules to his skin.

These handwavium glands which are close to the surface of the skin can then transfer stored potential energy into an object of choice. The object has to already be in motion, and the potential energy is added to it.

Elastic and inelastic collisions might help you build your handwavium glands. Also, the physics formulae around kinetic and potential energy aren't really hard to google.

This idea is very much like the X-men comics, in Gambit and Sebastian Shaw, so that might give you some ideas on where to go with it.

Remember, your story is always better served when you put an upper limit, well defined, to your character, otherwise, this combination is going to be seriously OP.

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A couple of stray observations:

He could create a very efficient spaceship

Although your stipulation that he stores the kinetic energy as some kind of internal energy before releasing it conserves energy, there's no way for it to conserve momentum without severely restricting his abilities. The basic reason for this is that energy is not direction dependent, while momentum is.

So, imagine our hero is floating in space and has a basketball with him, as one is wont to do in space. He can push the basketball away from him, and Newton's third law would kick in to propel him in the opposite direction. Now, for normal humans, you couldn't do much with this; once you push the basketball away from yourself, you can't retrieve it without undoing all the delta v you just accrued and so how fast you can ultimately go depends entirely on how hard you can push the basketball. To really overdo this analogy, normal rockets essentially get around this by carrying, in scientific terms, a lot of basketballs to push off the end of the ship.

Now, this is a problem because each basketball has some mass, so the more basketballs you have, the more massive your spaceship is, which requires more basketballs to accelerate it, which leaves it more massive, etc. When the dust clears, you're left with the Tsiolkovsky Rocket Equation, which is a major downer for interstellar travel and basically means that it's really hard to get a rocket moving very quickly.

However, our superhero can conveniently circumvent this painful dose of reality. For, after he throws the basketball away from him, he can use his powers to stop it relative to himself, use arbitrarily little energy to reel it back to himself, and then release his stored energy back into the ball to accelerate it away from himself again. By continuing this process ad infinitum (and perhaps using more efficient methods than a basketball), he has essentially created a reactionless drive and can achieve delta v's that NASA could only dream of. Every space agency in the world would be chasing this guy. Granted, this isn't directly a military use, but it's not hard to see how to make one from it (ie make the ships fire targeted, relativistic projectiles towards earth).

He could destroy absolutely everything

An interesting thing about energy is that although it is conserved in every frame of reference, the total amount of energy can vary from frame to frame. For instance, if our hero is falling towards Earth at terminal velocity, people on Earth will see him as having about $10^5 J$, so if he used his power and stopped himself from falling, they would reason that he could unleash $10^5J$ in whatever way he wanted. However, from our hero's perspective, the entire Earth is falling towards him at terminal velocity, so if he stopped the Earth he could unleash a mind-boggling $9*10^{27}J$ of Energy!

Now, to fix this discrepancy, we have to pick a certain reference frame in which we tally up the energy he is allowed to use, but the most natural frame is the one in which our hero is at rest-- and that leads to obvious problems, as seen above. And really, no matter what frame we use, we'll run into the problem that his energy storage capabilities are insanely overpowered. After all, what's stopping him from using his powers to halt the motion of the entire Andromeda galaxy relative to himself?

Of course, we could always impose arbitrary limits on how much energy he can store/how far away the object can be but where's the fun in that :)

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Tank gun and rifle speed

First you have to know that I will use this formula to calculate the kinetic energy of the projectile. $$F =mv$$

Where:

  • $F$ is the force in newtons.
  • $m$ is the mass of the bullet in kilograms.
  • $v$ is the speed of the bullet in metres per second.

And this to calculate the speed of the projectile: $$v = \frac{F}{m}$$

Also, I notice that there are a lot of 7.62 mm bullets, so I am not sure which use to do my calculations. You are talking about a soldier so I think it may be rifle... and the first link I found was .300 AAC Blackout -125 gr weight?-, please, if you know about what I am talking about... help me... :(

There are a lot of different tank guns, but I found this in Internet.
If you want, you can tell me an specific gun tank, it's ammo and/or rifle's ammo and I can make the calculations and add them to the table: $$ \begin{array}{cc|cc|c|cc} \text{Gun} & \text{Ammo} & \text{Weight} & \text{Speed} & \text{Force} & \text{7.62mm Speed} & \text{Mach}\\ \text{Rheinmetall 120 mm gun} & \text{M829} & 10,5 \text{ kg} & 1,670 \text{ m/s} & 17,535 \text{ N} & 140,280 \text{ m/s} & 412.22 \\ \text{""}& \text{M829A1} & 4.6 \text{ kg} & 1,575 \text{ m/s} & 7,245 \text{ N} & 57,960 \text{ m/s} & 170.32 \\ \text{""}& \text{M829A2} & 9 \text{ kg} & 1,675 \text{ m/s} & 15,075 \text{ N} & 120,600 \text{ m/s} & 354.39 \\ \text{""}& \text{M829A3} & 10 \text{ kg} & 1,555 \text{ m/s} & 15,550 \text{ N} & 124,400 \text{ m/s} & 365.55 \\ \text{""}& \text{DM33} & 8.35 \text{ kg} & 1,750 \text{ m/s} & 14,612.5 \text{ N} & 116,900 \text{ m/s} & 343.52 \end{array} $$ Like you can see, you soldier is overpowered, I would recommend add a limit of absorbed energy, a power conversion efficiency, or something like that. Obviously with this incredible speed for the proyectile it would be able to pierce a lot of armor (even the tank armor), I mean, we are talking about at least 170 times the speed of sound. (Use @James McLellan answer for a depper information about piercing).

Energy Storage

You said you want to make your ability a bit more "realistic" so I think I can help you understanding how to store the energy.
I based this part of my answer in this previously answer I made for a magical question...

  • Heat:
    Human body (warning articule in spanish) has a heat capacity of 0.83 cal/gr or 3,47272 j/gr.
    Knowing $\text{N} = \text{J}$ we could know that each newton is equal to a joule.
    And also we know that each $1 \text{ kN}$ absorbed by it's body he would heat (average $70 \text{ kg}$ human) $4.11 \text{ Cº}$. Congratulations! Now your ability is underpowered, at least if you don't want to cook your soldier...

  • Potential Energy:
    He could store energy like potential energy, I mean, each time he aborbs kinetic energy he increase temporaly its weight.
    We can't use newtons for this, because they doesn't count gravity. We have to use kilogram force: $9.80665 \text{ N} = 1 \text{ Kgf}$.
    That means, each $1 \text{ kN}$ absorbed by it's body he would weight $101.97 \text{ kg}$. Better? He won't die cooked, now he would die crushed or by a cardiac arrest.

  • Chemical Energy:
    This is the most interesting way of store energy. Basically it works combining chemical compounds (like 6H2O and 6CO2) into complex ones (like C6H12O6 and 6O2) and then uncombine them to get again the energy.

    • Glucose: I don't want to calculate the energy used to make glucose in photosynthesis, so I will use the output energy of "burn" glucose in our body. $16 \text{ kj/gr}$.
    • Water: based on that Chemistry SO answer we know that water has $285.8 \text{ J/mol}$ or $15.86 \text{ J/gr}$.
    • Fat:: Each grame has $37 \text{ kj/gr}$

    This method seem very well, $1 \text{ kN}$ absorbed by it's body he would produce $0.0625 \text{ gr}$ of glucose, $0.063 \text{ gr}$ of water or $0.027 \text{ gr}$ of fat. Not much, but you need the "resources" to make that molecules (O2, H2, CO2, H2O, etc) and maybe that could cause nutritional problems. $$ \begin{array}{c|c|c|ccc} \text{Ammo} & \text{Heat} & \text{Weight} & \text{Glucose} & \text{Water} & \text{Fat} \\ \text{M829} & 72.13 \text{ Cº} & 1,788.07 \text{ kg} & 1,095.93 \text{ gr} & 1,105.31 \text{ gr} & 473.91 \text{ gr}\\ \text{M829} & 29.80 \text{ Cº} & 0,738.78 \text{ kg} & 452.81 \text{ gr} & 456.68 \text{ gr} & 195.81 \text{ gr}\\ \text{M829} & 62.01 \text{ Cº} & 1,537.22 \text{ kg} & 942.18 \text{ gr} & 950.24 \text{ gr} & 407.43 \text{ gr}\\ \text{M829} & 63.96 \text{ Cº} & 1,585.65 \text{ kg} & 971.87 \text{ gr} & 980.18 \text{ gr} & 420.27 \text{ gr}\\ \text{M829} & 60.11 \text{ Cº} & 1,490.06 \text{ kg} & 913.28 \text{ gr} & 921.09 \text{ gr} & 394.93 \text{ gr} \end{array} $$

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  • $\begingroup$ Kinetic energy does not equal force....at all. Force equals mass times acceleration $F=ma$; and KE equals one half mass times velocity squared $E_{k}=1/2mv^2$. Please fix the physics in this answer. Also, if you really don't understand what the person is asking, then the question should be closed as unclear. Please don't answer in that case. $\endgroup$ – kingledion Dec 30 '17 at 1:29
  • $\begingroup$ @kingledion. A) Ok, I've edited my post, I made a mess but now I think the proper formula is with Force and not Kinetic Energy. B) I know he ask about effect of the increase force in the rifle bullet, but also I read he ask about feasibily and I wasn't 100% sure, but I post a help of how could he store all that energy inside its body. $\endgroup$ – Ender Look Dec 30 '17 at 1:51

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