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In the system of magic that I am using I try to follow the laws of physics as much as possible. The idea being that whatever magic can do, a scientist and machine can eventually do. One of the things my spellcasters can do is transfer and convert kinetic energy. I know there isn’t a scientific principle that allows instantaneously converting kinetic energy into heat but hey, it IS magic.

One of the fundamental rules of my magic is that it is impossible to manipulate any energy or atoms that are within 3-6 inches of a living person. The caster can't touch that space, but if they do something to the surroundings it can go into the space, ie, one can't push a rock while it is inside the field but can throw it into it. I don’t like mind control and that was the easiest fix. But it leads to an interesting complication which leads to my question.

Question

What would happen if a midsized sedan style car going say 60 mph had all of its kinetic energy stolen? Assume the time frame for this is about 1/1000th of a second (I do not know how fast this happens changes things, I want it close to instantaneous but not quite so that another caster can maybe interfere). Also assume that all the energy (minus the heat loss, since even in magic no transfer of energy is perfect) is completely transferred into the ground in that time frame.

Now how does that change when there is someone in the car? The person means that there is a small but noticeable part of the car whose kinetic energy cannot be touched by the caster at all.

Finally how much different would this be if rather than a big heavy car going fast, it was a light small bicycle going slow? The human's protection is much more of a bicycle’s total space.

Hard science with math being given would be beneficial but I’m not looking for how many joules or whatever the unit is will be applied to each atom of the person or whatever. Mostly I want to know how this compares to a normal car accident and in general terms what effects weight and speed and proportion of vehicle protected by a living person might have when all the kinetic energy is removed like this.

My answer

The situation described is like a car hitting a wall that doesn’t break, except that it's all of the energy from every spot leaving the car at once. I know in a crash, the impact point hits first and the back end follows fractionally later, hitting what already stopped. Seems like if all the energy was stolen at once, the car would come to a complete stop and remain intact because every part stops at once. The person inside though would still keep going, essentially crashing without airbags or crumple zones or most of a car's safety features. Instead of saving the driver, the caster killed them more horribly than the accident would have. With a bike, seems like it would be the same as hitting a curb dead on, and the thing would flip (the front tire being one of the few parts that isn’t protected).

I believe this is different from any other question because of the complication of the human's null field. If not, a link to some other question will be a good answer.

Edit to clarify

This is different from a head-on crash against an immovable wall because:

  1. the null field inside generated by the person
  2. the kinetic energy is stolen from every atom of the car at once, rather than when a car hits a wall and the impact point losing the kinetic energy before the points further away from the point of impact.

Second edit; I did not understand that kinetic energy was relative to a frame of reference so when I say all the kinetic energy is taken, I mean relative to the spell caster standing on the street. Its bad enough the spell intended to save a life, stopping a runaway car, brutally kills the driver without sending them into orbit after cooking them with atmospheric friction.

This is where my mental hang-up comes from.

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  • $\begingroup$ Most of what you want to know about the math is in the first three paragraphs of en.wikipedia.org/wiki/Kinetic_energy . Your physics seems reasonable. $\endgroup$ – user535733 Jul 5 '18 at 3:27
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    $\begingroup$ So, to clarify your clarification; what you're describing is still a massive momentum change (down to 0) with the car not being deformed in any way, AND the null field of the passengers effectively leaving their momentum perfectly intact, correct? I'm confused as this edit was made after I posted my answer but I'm not sure if you're suggesting that this makes my analysis invalid or not, and if so, in what way. $\endgroup$ – Tim B II Jul 5 '18 at 3:51
  • $\begingroup$ @TimBII i wasnt sure if the answers were reflecting that tge KE is being removed from the entire car (minus the null field) or if they were just treating it as a standard head on collision. I believe this difference is important, but if i knew for certain wouldnt need to ask. Does not invaladate any answer, but wanted to emphasize the most important parts for future answers $\endgroup$ – Seserous Jul 5 '18 at 3:58
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    $\begingroup$ Good point, but yes, these answers are pretty much covering that. Thinking of it another way, if a sledge hammer hits a wall and the wall holds, the sledge has not deformed, but its kinetic energy is entirely absorbed by the wall. In your case, it's absorbed entirely by magic. This is important because in a modern car, some of the energy of a collision is absorbed by the deformation of the car itself whereas your magic actually treats the car like it's non-deformable, potentially killing the passengers without triggering a single safety response. The only damage it receives is blood stains $\endgroup$ – Tim B II Jul 5 '18 at 4:06
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    $\begingroup$ if it is ALL kinetic energy then about half the time the car goes rocketing up into the sky, or more correctly the earth moving at millions of KM an hour away from it an the car falls away. If the effect wears off fast enough then the car comes crashing back down to the ground. $\endgroup$ – John Jul 5 '18 at 18:48

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This is exactly what used to happen to cars before Volvo introduced Crash Test Dummies

When I was growing up, many of my parents' friends were constantly bemoaning the fact that cars 'just weren't built the way they used to be' and a big part of that was that they just didn't feel safe in cars that weren't built to be sturdy; Solid chromed bumpers, very rigid thick metal chassis and doors, et al. Of course, what we now know is that the cars they were talking about were in reality death traps.

Volvo introduced the concept of testing cars in crashes, and the effects on people using crash test dummies. Pretty soon after doing this, Volvo cars were released into the market with 'crumple zones' that allowed for the kinetic energy to be dissipated before reaching the passengers. The idea is that if the cars hit something, the kinetic energy would be absorbed by the crumple zone and the relative impact of the momentum change on the driver and passengers would be reduced.

We now take that to even further extremes with airbags, engine mounts that drop the engine out in the case of accident, seat belts, etc.

Ultimately, what you're describing is exactly what used to happen when old heavy rigid cars hit walls, or each other. The kinetic energy of the car was immediately reduced to zero, but the kinetic energy of the passengers was not.

The result? Death, mostly. People hitting steering wheels, windows, gear sticks, etc. at 100 km/h (~62 mph). These cars (despite the perception among their drivers) were inherently unsafe. Ultimately, stealing a car's kinetic energy instantly can be done through magic, or just putting a wall in front of it. In both cases, for the sake of survive-ability you want the car to be the most deformable object in the collision, not the passengers.

Your current model has exactly the opposite happening. It's actually an effective way to either seriously damage or even kill people in an otherwise safe car.

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    $\begingroup$ Volvo? GM did the first crash test and produced the firs crash tests dummies. $\endgroup$ – Nathan Cooper Jul 5 '18 at 9:12
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    $\begingroup$ This link shows a poor quality video of an old, rigid car crash. $\endgroup$ – Bernat Jul 5 '18 at 9:36
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    $\begingroup$ Even more illustrative is comparing early Formula 1 crashes to today. Basically, in the early days, the car would be almost unscratched, but the driver was almost certainly dead or at least heavily injured. Today, you need a vacuum cleaner to pick up the car, so little is left of it, but usually, the driver walks away from the crash on his own two feet. $\endgroup$ – Jörg W Mittag Jul 5 '18 at 10:56
  • $\begingroup$ @Bernat It's a shame I can't watch those videos without first downloading them to my computer and opening them in a separate app... $\endgroup$ – Michael Jul 6 '18 at 23:24
  • $\begingroup$ @Michael there are browser addons that simplify that task, allowing you to right-click and open the video on some external player without having to download it first. $\endgroup$ – André Paramés Jul 8 '18 at 13:48
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What everybody already said about death due to collision with the dashboard and steering wheel, plus the seatbelt issue... Well, kinda.

Don't forget that temperature depends on the kinect energy of atoms. By sapping practically all of it from the car, it will be just a little above absolute zero (it will still have some energy from the heat transfer). The various components of the car and the fluids and fuel in its reservoirs and engine will go from solid, liquid and gas states to different, interesting states of matter, so it will probably cease being a car in fractions of a second. I don't have a properly equipped lab nor a disposable car to test, but I think the car would fall apart before the driver woud die from the impact. They would still impact against something not made for safe impacting, though, and depending on what the car transmogrifies into, it might become a non-newtonian fluid, or a very hard solid block mostly shaped like a car.

There is one very interesting scenario. By freezing the molecules so fast, the molecular bonds may be broken. Some things will become superfluids at close to absolute zero... Long story short, if the car as a whole became a superfluid, the driver (who would keep their momentum) would fly through it like a ghost. They would then die by impacting against the asphalt at great speed while their whole body surface is enveloped in a very thin coating of a liquid at a ~0 Kelvins temperature. If they do survive the impact, the liquid will creep into their eyeballs, lungs, skin pores and intimate parts, killing them from suffocation and a case of freak hypothermia.

Meanwhile, you say that the energy of the car is transfered to the ground below. We are talking about taking the thermal energy of a 1 to 2 tons object and transfering it all to the ground.

The specific heat of asphalt is almost double that of steel. To simplify things, let's consider specific heats are constant and independent from temperature, that the car is made completely of steel, and let's do some rounding. If you are transfering from the car to an equal mass of asphalt, for every two degrees the car cools down, the asphalt heats up by one. This will happen with any scale, i.e.: Kelvin, Celsius or Fahrenheit. If the car cools down by 300K (about ambient temperature in many places), the asphalt will heat up by 150K. If you stood barefoot on it you might get 2nd or 3rd degree burns on your soles. If you are transfering just to the asphalt in contact with the tires, the asphalt will probably melt. Depending on its quality, it might have enough trapped gas in it to cause it to explode.

Since the ex-car will be on top of it, the hot asphalt will exchange heat with the bose-einsten condensates, superfluids and whatever. This will also probably cause explosions, strong enough to kill anyone close to it.

All in all, I think that usage of your magical spell as a brake mechanism for cars should be considered unsafe by transit authorities worldwide.

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    $\begingroup$ Does any of this very interesting answer change with a reference frame of someone standing still on earth? Then the car still has kinetic energy of being attached to earth so are the atoms still going to near absolute zero? $\endgroup$ – Seserous Jul 6 '18 at 1:16
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    $\begingroup$ @Seserous the kinect energy of atoms that give them temperature is not related to the reference frame regarding an external observer. Temperature is not their kinect energy relatove to Earth, it is the average kinect energy of each and every atom in relation to the car itself. $\endgroup$ – Renan Jul 6 '18 at 1:33
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    $\begingroup$ As long as we're taking this to the extreme, wouldn't taking away all kinetic energy also cause the car to either be flattened by the planet or fly off the face of the planet, depending on whether it is on the side the planet is moving towards? It would also seem to imply that the person would hit the part of the car in the direction of the planet's orbit before they had a chance to hit the windshield (if the direction isn't relatively forward). $\endgroup$ – IllusiveBrian Jul 6 '18 at 15:15
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    $\begingroup$ @IllusiveBrian I was thinking about taking away all the kinect energy relative to the caster (effectively halting the car) and then the kinect energy from its atoms (which does not affect the car's speed). I am not considering removing kinect energy from any other frame of reference. $\endgroup$ – Renan Jul 6 '18 at 15:24
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    $\begingroup$ If the superfluid film is so thin, then shouldn't heat transfer from the driver's body and the surrounding air cause it to exit the superfluid state long before it would suffocate the driver? $\endgroup$ – Aidan F. Pierce Jul 7 '18 at 23:02
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Your person will die, but not because they are colliding with the car. It would actually be the seatbelt that kills them. The few centimeters around the person move, but the seatbelt won't extend because its on a reel and you have stopped that part from moving. When the car stops your person should move forward into the dashboard and basically die, but the seatbelt will hold their body in place. The only problem is their head will keep moving forward and with such a huge amount of G's it will likely break their neck which would kill them.

I put the numbers into the website with a G force calculator. https://rechneronline.de/g-acceleration/ 60mph to 0 in 0.001 seconds would result in -2700Gs which is higher that the maximum G-Force tested of 46.2 G's (and that was in the opposite direction). So its pretty safe to say they are dead.

So based off that, I'm not sure if you will break your neck first, or your seatbelt will fail and send your body and lowered head into the dashboard which would then kill you. Either way you've got a dead person.

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  • $\begingroup$ What happens to the rest of the car after the person is dead? $\endgroup$ – Seserous Jul 5 '18 at 3:32
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    $\begingroup$ @Seserous You might get some tearing around the seat and steering wheel since its kept moving while the car is stopped. The car itself should be stopped but it might roll forward slowly from the persons impact with the dash. $\endgroup$ – Shadowzee Jul 5 '18 at 3:37
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    $\begingroup$ @Seserous In a motorbike, the person would be launched forward out of the bike and their body would deflect off the bike as they moved forward sending them cartwheeling away at 60MPH. Not sure if they would survive or not because it would depend on how they stop moving. $\endgroup$ – Shadowzee Jul 5 '18 at 3:40
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    $\begingroup$ @Seserous The Expanse Season 3 happens to cover this exact topic. What it depicts is exactly in line with this answer. Here's the relevant clip: youtube.com/watch?v=waG8YYTwpAQ $\endgroup$ – nullpointer Jul 5 '18 at 4:14
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    $\begingroup$ If you are unfamiliar with The Expanse, be aware that the clip linked by @nullpointer is rather graphic. $\endgroup$ – chepner Jul 6 '18 at 3:52
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The car part of the question has been answered quite well, but I think I can add to the bicycle part.

If I understood correctly, your spell is stealing the kinetic energy from the targetted object but does not actually hold it in place. When targetting a car, this point is irrelevant, but bicycles usually weight a lot less than the rider. So in this case part of the bicycle "stops" when kinetic energy is stolen, but at the same instant collides with the still moving parts and the rider. This could be compared to taking a running start and jumping on the standing bicycle or having a weak rope attached to your bike that snaps when it becomes taut. This would of course throw an unprepared rider off balance but not necessarily result in a crash. And certainly not the scenario, in which the bicycle stops dead and the rider cartwheels forward (though motorbike is closer to that since it is usually heavier than the rider)

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  • $\begingroup$ In other words: a very funny way to prank people :D $\endgroup$ – Renan Jul 6 '18 at 1:40
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The same thing that would happen if the car had all its kinetic energy transferred into a cliff by crashing into it. What would survive would depend on how long the transfer took. Also, whatever the kinetic energy gets transferred to has exactly the same problem (although it could be better-prepared to deal with it).

If you’re trying to stick to real physics, remember that there is no such thing as an absolute frame of reference. You don’t have “zero kinetic energy,” or “lose all your kinetic energy.” You are at rest in some frame of reference. In everyday life, usually think of that as the inertial frame of reference of whatever we’re standing on, or possibly that of the surface of the Earth where we are if we are currently both moving relative to it and looking out the window. That kind of intuition is not actually logically consistent.

If something else is “stealing” the kinetic energy from an object, the relevant frame of reference might instead be whatever receives the energy, or whoever is casting the spell. Any of those possibilities open up possibilities for clever shenanigans.

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  • $\begingroup$ +1 for the idea that completely stopping the car relative to all reference frames would include its massive reverse acceleration as the other objects of the spinning Earth plow into it at about 1000 mph. $\endgroup$ – lly Jul 9 '18 at 3:31
  • $\begingroup$ ...although @Demigan's answer below does pay more attention to those effects and some of the reference frames that might be involved. $\endgroup$ – lly Jul 9 '18 at 3:41
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The kinetic energy disappears into another medium. It doesnt flow into another object so theres no collision damage like you already suggested. Unfortunately there are two problems, lets start with the smaller one:

The guy in the car prevents his kinetic energy from being taken (and part of his chair), so while the car stops the guy is still going 60mph. Normally the crumplezone would help the guy slow down but that doesnt happen, and without the shock of a normal collision the airbag won't deploy, so basically he hits the dashboard&seatbelt with 60mph and probably dies.

Now to the bigger problem: the earth is moving, 30km/s compared to the Sun, 300something km/s around the universe center, and about 600km/s compared to a "rest frame". You take ALL its kinetic energy, it means that its either going to smash into the earth or the atmosphere at ludicrous speeds. Being LAUNCHED into Saturns storm from orbit is friendlier than having all your kinetic energy leeched. Theres also the question of where that much energy goes if you convert it, it had better be a good place to store it. So I hope the mages use themselves or the center of the earth as a referenceframe to avoid this. Even so, problem number 1 will still kill you.

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    $\begingroup$ This answer deserves more love. Humans are fundamentally terrible at considering the incredibly fast moving reference frame we're all bound in on earth even when ''stationary''. $\endgroup$ – Smeato Jul 5 '18 at 13:06
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    $\begingroup$ But kinetic energy is not "actualized" - it occurs only in the case of impact (or, more accurately, when accelerating/decelerating), it is only a valid concept in the relationship between objects, not in itself. There is no ultimate "rest frame of the universe"; Einstein teaches us that all frames are made equal. OP's formulation to "steal all kinetic energy" makes no sense; what he, by his description means is "decelerate to the surrounding local frame of reference in 0.0001s". Hence, the 3rd paragraph is objectionable; if I would read such a description in a book, I'd probably put it away. $\endgroup$ – AnoE Jul 5 '18 at 16:30
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    $\begingroup$ @AnoE this is actually still being investigated: springer.com/gp/about-springer/media/research-news/…. You could argue that because light propogates independently from the movement of the object that there is a restframe: if light expands in all directions equally fast compared to the object despite the independent propogation it is at full rest. Regardless of the outcome of the research, the frame of reference matters a lot to the outcome of the question. Earth, Sun, universe, universal restframe if it exists... $\endgroup$ – Demigan Jul 5 '18 at 18:24
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    $\begingroup$ Well, sure, there is always new knowledge, but the question strikes me a lot as having the meaning I mentioned (local frame of reference in a "canonical" way, i.e. in the case of a car the street (or, equivalently, the planet) would be the one). $\endgroup$ – AnoE Jul 5 '18 at 18:28
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    $\begingroup$ @AnoE The reference to "all kinetic energy" is a large one and regardless of canon, you need to be aware of the reference frame. It's undeniable that the earth is moving, and there are many reference frames that could be picked. The "canonical" could also be the earth or the mage, and if the mage is moving in say an airplane and uses in on the Car... $\endgroup$ – Demigan Jul 5 '18 at 19:33
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The momentum theorem states that $m\times\delta v = F\times \delta t$, or $ F = m \times \delta v /\delta t$

Putting in that formula your numbers, we get $ F = 1300kg \times \frac{27\frac{m}{s}}{0.001s} = 35.1 \times 10^6 N$

That's a huge force, and that's what normally happens when a car hits a hard and non movable obstacle. Maybe quicker.

What kills you is not the velocity, but the sudden deceleration

All that kinetic energy will be converted to plastic deformation and then to heat.

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The structural integrity of the car should remain intact. If all the mechanical parts come to a complete stop at the same time, they shouldn't take too much of an offence at being interrupted so rudely.

Airbags would not go off, that is unless your magic can spare the accelerometers that triggers them. On the other hand, seat belts can be triggered by acceleration of the car and/or something hitting the belt hard enough. My belt triggers when I jerk in my seat at a red light, 96 to 0 will do the trick just as well. Obviously, that assumes your world has (functioning) seat belts, otherwise you are done goofed.

Anything not attached in the car will fly off at 96 km/h and will die and/or kill anything they come in contact with. Even a soft pillow will kill you, PSA: that is why you don't put loose stuff on your back seats.

Anything attached in the car will still hit something (namely, the seat belt) at 96 km/h, which means serious injuries up to death. The main issue is that your body is floppy enough that some parts will not be affected much by the belt. While your ribcage will be crushed by the impact, the head will tend to fly off. At 96 km/h, I wouldn't want to be your neck.

Your saving grace here would be hitting the airbag (which would stop your head from getting ahead of yourself too much, if they worked) or, probably more reliable anyways, neck braces like ones used in racecars. That will alleviate that problem enough to consider survival and going on with this answer.

Then how well the passengers will fare will depend on them physically, on how they put their seat belts on (also PSA: there is a right way and many wrong ways to do it), the speed of the "crash", how they are seated, etc.

One small last detail is the car dead stops, but not the ones behind it. When you hit a wall, usually other cars don't follow you, walls rarely occur on the road. There will be accidents behind you, although granted, you would have caused accidents anyways if you allowed your car to crash in the middle of the road.

But like Tim B II brilliantly noted, modern cars are designed to crash gracefully. It's probably a better idea to let good ol' engineering do the job rather than magic on that one. Also brilliantly noted accelerometers are part of the car, which matters for airbags.


For a bike, you might still fly off the bike, and provided you don't hit the bike (or something else) on the way, you might skip off the road. I have one word to describe that: ouch.

Your survivability will depend mostly on how well you are equipped. You would probably want to wear the same equipment motorcyclists wear (or at least should wear, they don't all do and that's on them): helmets, gloves and vests with padding to prevent scrapping your skin on the concrete.

On a bike, I don't think you are at a particular risk of death (unless you land on your neck, and other bad falls) if you at least wear a helmet. Other injuries won't be as egregious as it would be on a motorbike, but I would still advise you to wear whole shebang if you don't like the idea of severe burns on your skin.


If Ek = m/2 * v², another possibility is the car losing its mass rather than its speed. I imagine it would look like a flash of light, because A) light has no mass allegedly and B) it'll look hella cool, but that's pure speculation. But imagine the passengers flying off in the air and landing on their arse on the ground, how hilarious (aside from the burns from sliding on the ground that is).

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    $\begingroup$ Actually no, the accelerometers for the airbags would not trigger at all. Accelerometers work by measuring change in kinetic energy by measuring pressure on a spring (more or less) but if all the kinetic energy is removed apart from that of the human, then the mass in the accelerometer will also have its energy removed, meaning the car will experience a complete stop without accelerometers even noticing. To them, the car was never moving because all the energy has been removed, including that of the springed mass. $\endgroup$ – Tim B II Jul 5 '18 at 8:45
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    $\begingroup$ @TimBII That's an oversight on my part. Maybe they should make magic-proof accelerometers though. $\endgroup$ – AmiralPatate Jul 5 '18 at 9:38
  • $\begingroup$ Actually that's a good observation. Perhaps the 'springed mass' inside the accelerometer could be a live ant or something... $\endgroup$ – Tim B II Jul 5 '18 at 11:12
  • $\begingroup$ Some car parts would be bothered by sudden stops. The piston assembly (I don't know the proper terms in English) might go of sync and break. $\endgroup$ – Renan Jul 5 '18 at 12:16
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Most answers here are focused on 'stealing' kinetic energy by removing the velocity; consider that kinetic energy could also be stolen by negating the mass of the object. This mechanism would have the advantage of consistently stealing kinetic energy in all reference frames, rather than just that of the magician or the road.

Immediate Effects

The car's occupant now dominates its mass. Rollover becomes the greatest risk without the bottom-weight of the engine and chassis. Wind risks also increase, as the car has the same surface area with much less inertia and gravitational force to hold it to the road; at high speeds, the car may even tumble end-over-end. On the bright side, skid risk should not increase, as the loss of mass (and gravitational force) decreases friction and inertia proportionately.

Collision

If a collision were sustained in this state, whiplash risks are increased without the big low-pass filter of the car's mass, and crumple zones would not function correctly without the cars mass to crumple them; harnesses and airbags would work as usual, and the risk of injury from the airbag's rapid deployment itself may be negated entirely.

The Bike

The biker would likely lose balance and fall due to the sudden loss of gyroscopic stabilization from his wheels.

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  • $\begingroup$ Welcome to Worldbuilding, mrum! If you have a moment please take the tour and visit the help center to learn more about the site. You may also find Worldbuilding Meta and The Sandbox (both of which require 5 rep to post on) useful. Have fun! $\endgroup$ – FoxElemental Jul 5 '18 at 20:14
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While all the answers reflecting on the state of the driver when they find their environment suddenly moving at -60 mph relative to their reference frame certainly have their merits, that is, in all likelihood quite dead after impact with said environment, I would like to reflect on the fate of the magician.

By the law of conservation of energy you cannot simply steal energy, it has to go somewhere. As it is the magician exerting this force I would venture that it is the magician who will have to absorb the stolen kinetic energy. This can be a combination of conversion to heat (of the magician) or kinetic energy (of the magician). The amount of energy to absorb is:

  • E = 0.5 * mcar * speedcar2 ~ 5 MJ (for a car of approximately 1300 kg driving at 60mph)

Transfer of the kinetic energy of the car to kinetic energy of the magician (relative to ground) will propel the magician away from ground at:

  • vmagician = sqrt( 2E / mmagician) which for a magician of approximately 80 kg comes to ~ 240 mph in the same direction as the car was moving (conservation of momentum) or, if using devices such as a seesaw, in another direction. Probably not a good idea.

Transfer of kinetic energy of the car to heat will warm up the magician considerably.

  • assuming the magician is a sack of water of 80 kg, which is a good enough approximation and also assuming a uniform distribution of the heating up throughout the magician, this comes to a rise in temperature of T = E / (Cwater * mmagician) = 5M / ( 4184 * 80 ) ~ 16 degrees centigrade. As the brain stops working when it gets too hot (and 16 degrees would certainly qualify as too hot), this is probably not a good idea either.

So, if killing the driver was not enough already, said spell will most likely kill the magician too, unless the energy was diverted elsewhere where it is dissipated harmlessly such as, say, increasing the heat (background temperature) of the universe as a whole.

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  • $\begingroup$ Hello, GretchenV, and welcome to Worldbuilding. Please take our tour and visit the help center to learn more about the site. Have a nice day! $\endgroup$ – Gryphon Jul 6 '18 at 12:08
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A modern 3-point seatbelt relies on deceleration to trigger the locking mechanism on the reel. It has a little Pendulum in the mechanism which detects sudden changes of motion.
In your Arrested Momentum scenario, the reel and pendulum are entirely locked up for the duration of the deceleration (more or less instantaneous) but critically, the movement forward of the person will not trigger the pendulum mechanism. meaning the seatbelt flat out will not restrain them at all..
So your driver and any passengers will fly forward at 60mph with hardly any resistance.

They'll hit the dashboard in a fraction of a second. The Airbag will not be coming to meet them the other direction, so they smack full-force into the dash and break their knees, arms and probably ribcage.

Most likely they'll get hurled headfirst out of the vehicle via the windshield, but they're still in their harness, so they'll go as far as the reel will allow them and then jerk around into a tumble when the seatbelt reaches its limit.
Expect broken bones wherever the seatbelt was in contact at this point.

They'll be dead from blunt-impact trauma and found mangled many yards down the road. Immediate cause of death will be a broken neck and skull-trauma from going through a windshield at 60mph without any restraints.

Needless to say any loose items in the car will also do this and there'll be a spray of debris in front of the vehicle too.

Not pretty.

On the other hand, this is contingent on the reel/pendulum mechanisms being located outside the bubble of anti-magic around the driver.

If the bubble encompasses the pendulum, your seatbelt will work mostly as intended even if part of the belt is outside it. The actual belt movement happens after the momentum is robbed, so the only part that matters is the pendulum.

In this case expect the passenger to instead die of a broken neck from the whiplash as their seatbelt holds their torso in place and their head snaps forwards. Again no airbag or any other safety equipment will be working.

Surviving either scenario would be downright miraculous and they will need immediate medical care.

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  • $\begingroup$ Whiplash is your head being jerked back (which it isn't designed to do). Heads move forward (and rotate down), just fine. This is pretty much how old-fashioned (non-retractable) seat-belts used to work. $\endgroup$ – Martin Bonner Jul 5 '18 at 12:41
  • $\begingroup$ Good point, though I can't imagine suddenly tipping forward at 60mph could be good for your neck either. $\endgroup$ – Ruadhan Jul 5 '18 at 14:04
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Amusingly if the magic shield boundary happens to intersect atoms (which I think is likely though I haven't done the math), your scenario of part of the vehicle being stopped would result in a rather dramatic nuclear fission all around the driver, possibly releasing vastly more heat than is safe for humans.

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Now you're playing with portals

Build a plywood wall in the middle of a bobsled run, at a point where any competent sledder will be going 88 mph. Paint it with moon dust, Sherwin Williams Decora Plus H2 Pastel Base, or any other coating that will accept portals. Open a portal there.

Take a semitrailer. Build a flat surface on it going essentially all the way to the ground. Also paint it with a portal surface. Open a portal there.

Have the semi drive 88 mph steady on. Send a bobsled down.

Boom, the bobsled is sitting in the middle of the highway completely stopped. Its kinetic energy has been stolen. The operators feel no acceleration or jerk. It just works.

You could do the same thing with a speedboat and an airplane in distress, have the speedboat tear down a river at 90mph whilst the airplane flies into a mountain painted with a portal at 130mph. Suddenly the airplane pops out over the river going 40mph, instantly suffers an aerodynamic stall and falls a few feet into the river, cushioned by the river.

With that concept, and the idea of manipulating the frame of reference, you should be able to figure out other ways to steal (cancel) kinetic energy in your magical system. Essentially you are sweeping the reference point such that the desired kinetic energy is canceled.

Rotational force is a problem. My first draft used a car. The car's linear velocity works fine, but there's a big problem with the rotating velocity of the car's monkeyworks. That would suffer extreme acceleration and jerk forces from the rapid change in rotational speed. So I switched it up to a bobsled.

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  • $\begingroup$ Is this like driving a truck foward at 60mph and shooting a tennis ball backwards at 60mph and watching the ball drop stright down because its velocity becomes zero? $\endgroup$ – Seserous Jul 7 '18 at 1:42
  • $\begingroup$ @Seserous I've never tried that and not sure it works. The portal would be a different effect, I think. $\endgroup$ – Harper Jul 7 '18 at 1:45
  • $\begingroup$ it works, saw it on a science show, think brain games. In your portal example, both objects going in the same direction? $\endgroup$ – Seserous Jul 7 '18 at 1:50
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The result should be almost the same as if the car had never been moving in the first place, but the passenger comes flying along from behind the car at 60mph and (after "ghosting" through the back part up to and including his seat) crashes into (the front part of) the car. This is far from healthy.

Strictly speaking, we have to do as if the car parts 3 to 6 inches around the passenger fly with him, but that won't make a difference for the person. The only difference is to the integrity of the car (or should I say the composition of its debris?), depending on how "soft" the field fades out with distance from the person (if the transition is atomic-level "hard", the parts will essentially break from the main vehicle as if they had never been joined before; if the transition is very soft and the parts deformable, they may end up brutally torn apart or "merely" deformed as if an unstoppable heavy tank ran into part of it at 60mph ...

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There are 3 impacts in collision. Your vehicle stops moving. The human collides with the vehicle/asphalt.

And your organs hit inside your ribcage/cranium. This compreses your hearth and lungs vs the sternum and ribs. Also your brain ricochets inside the cranium against the bone.

Here is why even if you look Okay from outside, your condition could be critical.

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I think the car would instantly disappear. The car would instantaneously reach absolute zero.

The 60 mph deceleration of the driver is nothing compared to the 10,000 mph (or higher) instant deceleration relative to the solar system, and the galaxy. Instant death of the driver.

The car would instantaneously approach some percentage of the speed of light relative to other nearby objects, which would mean that time inside the car could be affected.

The mass of the car would simultaneously increase exponentially because of the high relative speeds

I think the car would fragment on a microscopic scale and if the driver somehow miraculously survived the deceleration, they would immediately find themselves in the vacuum of space surrounded by a dust cloud of microparticles of what’s left of the car at close to absolute zero.

The infinite mass dust cloud would create a black hole (hopefully far enough away from our galaxy) sucking the driver in.

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