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If a jet engine is bolted to the equator near ground level and run with the exhaust pointing west, does the earth speed up, albeit imperceptibly? Or does the Earth's atmosphere absorb the energy of the exhaust, and transfer it back to the ground, canceling any effect?

Urb
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FlanMan
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    Are you asking about the state when the engine is running, or are you asking about the state a considerable time after it has stopped? (The answers are probably different.) – Peter - Reinstate Monica Jan 18 '21 at 11:09
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    Perhaps clarify if you treat earth and air as a system. – Alchimista Jan 18 '21 at 11:55
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    See also the Eötvös effect, an object moving East vs West changes it's weight; none of the answers calculate the wobble imparted by applying force to the edge of a sphere versus an internal mechanism that applies force to all edges; possibly imparting a minutely tiny Wankel style effect. – Rob Jan 20 '21 at 19:45
  • and the International Earth rotational references: https://www.iers.org/IERS/EN/DataProducts/GeophysicalFluidsData/geoFluids.html and https://www.science.gov/topicpages/f/frequency+earth+rotation.html - heating will cause lighter than air gases to escape. Even with a closed system of the Earth and Moon the increase of the distance from the Moon causes a measurable change each year. – Rob Jan 20 '21 at 20:34

12 Answers12

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It's the latter. Look at the system earth + engine + atmosphere. Conservation of angular momentum must hold for the whole system (assume no gases leave the atmosphere due to the engine, which is a fair assumption).

noah
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    an answer that mentions gases leaving/not leaving earth's orbit. (atmosphere) Tick. – phil soady Jan 18 '21 at 08:08
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    The OP, I believe, is not asking about the overall system. They are asking about the solid sphere (they oppose "earth" explicitly with "atmosphere"). The angular momentum is constant for the overall system, but that is a trivial statement. – Peter - Reinstate Monica Jan 18 '21 at 11:10
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    @Peter-ReinstateMonica You seem to be suggesting that the engine could impart an overall net movement of the whole atmosphere with respect to the solid part of the earth. That's quite a powerful engine you got there, making the assumption of no gases leaving the atmosphere unreasonable and the trivial statement wrong. – noah Jan 18 '21 at 11:25
  • @noah But any jet engine, independent of its power, "impart[s] an overall net movement of the whole atmosphere", as long as it's running, and for a while after that, until all directed movement of the atmosphere has ended up in turbulence and, eventually, heat. It even happens when I blow air from my mouth. It is, of course, immeasurably little (angular) momentum, but it is there. Yes, the whole time the (angular) momentum of the overall system (Earth + atmosphere) stays constant (that's trivial); but it distributes differently between Earth + atmosphere. – Peter - Reinstate Monica Jan 18 '21 at 12:34
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    @Peter-ReinstateMonica That's not necessarily true. I was always assuming a steady state (i.e. not immediately after turning on the engine), sorry if that caused some confusion. It is perfectly possible to construct a setup in which all the momentum produced by the engine is cancelled locally, leaving no net angular momentum change. The point I was making was more of a view on the dimensionality of the problem. The engine will not cause the whole atmosphere to rotate with respect to earth, all effects will cancel more or less locally. – noah Jan 18 '21 at 12:44
  • Hmmm... so you think the steady state will be a perfect circulation where all of the exhaust feeds back into the intake (locally, without circling the Earth)? Interesting. I'd still argue that you'll create some overshoot in the jet direction, an "overall net movement". If we mentally eliminate air viscosity and friction between air and surface that would certainly be the case; friction just dilutes the effect, as always. So if with a stronger engine and less friction you could move the Earth, you'll have the same effect in real life, just smaller. – Peter - Reinstate Monica Jan 18 '21 at 13:29
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    I'm not sure "dilute" is the right word here. If I built a rail track all around the equator and pushed a cart down that track, I can argue that without any friction it will come back the other side. If I look at the problem with friction, it doesn't just "dilute" that observation, the cart will simply not return. – noah Jan 18 '21 at 13:36
  • Hm. Interesting. Maybe. True with the cart, not necessarily with fluid dynamics (where friction goes to zero with diminishing speed). Vortices etc. travel long distances. Such small ones not necessarily around the Earth though, true. Hm. – Peter - Reinstate Monica Jan 18 '21 at 15:00
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Briefly, the Earth's rotation rate will change; and the net rotation rate of the atmosphere will change in the opposite way. But very soon, friction between the air and the ground will ensure that the air and ground have no net rotation difference, which means that the Earth's rotation will return to the original rate.

S. McGrew
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    Compare and contrast: blowing on the sails of a ship you're currently standing on and expecting it to move, or yanking really hard on your boot-straps and expecting to get off the ground. There may be some inertial effects briefly resulting in movement, but it'll all cancel out when all is said and done. – Ruadhan2300 Jan 18 '21 at 08:44
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    In the spirit of "but what if ..." you could maybe consider a really powerful engine at the top of a high mountain and able to propel some gas right out into orbit, or even to escape velocity ... – Andrew Steane Jan 18 '21 at 09:28
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    In fact, the solid Earth and its atmosphere do have some natural rotational difference (tides, solar winds, convection, etc...). – fraxinus Jan 18 '21 at 09:51
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    @ruadhan2300: It is actually possible to "blow your own sail." It is inefficient, but it does work. – JRE Jan 18 '21 at 10:27
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    @JRE - That particular video is not very illustrative because their setup is inherently unstable and they obviously have no concept of how real-world sailing happens. They could do much better if they used the Bernoulli effect. – chasly - supports Monica Jan 18 '21 at 10:56
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    @BlueRaja-DannyPflughoeft Well, only if matter is driven into orbit or beyond. Is that what you mean? That's clearly not the scenario the OP had in mind, so that this answer is correct. – Peter - Reinstate Monica Jan 18 '21 at 11:07
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    Or if the engine is so powerful that it's able to create a net wind that goes all the way around the Earth – user253751 Jan 18 '21 at 13:39
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    Even that wind will slow and eventually stop due to friction. – S. McGrew Jan 18 '21 at 14:11
  • @user253751, The answer already acknowledges that case where it says, "...net rotation rate of the atmosphere..." – Solomon Slow Jan 18 '21 at 14:25
  • @S.McGrew When you turn off the jet engine, yes... – user253751 Jan 18 '21 at 14:42
  • @Ruadhan2300: Blowing on your own sail would work, if only you could blow hard enough. This is essentially what the thrust reversers on jet airplanes do: they blow against metal deflectors that turn the jet exhaust 180 degrees in order to slow down the plane. – jamesqf Jan 18 '21 at 17:29
  • @Peter-ReinstateMonica It's not actually possible to reach orbit from a point within the atmosphere with a single impulse. So, even if the exhaust achieved orbital velocity, it would still dip down into the atmosphere at some point and transfer its energy back to the planet. So the only way to permanently change the Earth's rotation rate is if the exhaust was at or beyond escape velocity at when it left the atmosphere. – HiddenWindshield Jan 18 '21 at 21:41
  • This (like all the other no-net-change answers) forgets about the chemical energy of the fuel. – philipxy Jan 18 '21 at 22:08
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    @philipxy No, they don't. While you can convert chemical (or nuclear, gravitational, whatever) energy into kinetic energy, e.g. by spinning up two opposite rotating wheels, you cannot change momentum or angular momentum of a "closed" system. – Peter - Reinstate Monica Jan 18 '21 at 22:34
  • The particles in the fuel & oxidizer remain closer together than before. The system has that permanent chemical-kinetic change. Along with increased angular momentum. (We'll have to disagree.) – philipxy Jan 18 '21 at 22:40
  • @Peter-ReinstateMonica - You don't need to have a plume of matter ejected into space. Some of the energy transferred from the engine to the atmosphere will be lost to space (radiated away as heat and, if the engine runs hot enough, light; and possibly other things if the engine runs ludicrously hot). It's a hugely small amount, but then so is any change in the earth's rotational speed arising from a jet engine bolted to it, and so is any differential between the earth's speed and the atmosphere's, and so is any amount of friction caused by that minute difference, etc.. – aroth Jan 18 '21 at 23:28
  • @aroth, energy is not the conserved quantitiy of interest, that is (angular) momentum, and momentum is not easily lost to space (excluding tidal effects) – Jasen Jan 19 '21 at 09:17
  • if you blow on the sails of a ship your standing on, it does move. this is a common and well-understood aspect of boating physics. – Fattie Jan 19 '21 at 14:17
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Gedankenexperiment

Suppose you have a large torus floating freely in space. The torus is sealed and full of liquid. Inside the torus is a propeller (or screw, if you prefer). I believe your question is reasonably equivalent to:

If the propeller in the torus starts spinning, will the torus experience angular acceleration?

Hopefully it is obvious that the propeller will accelerate the fluid in one direction, and that this will impart angular velocity to the torus in the other, thus causing it to spin [faster]. At the same time, once the propeller stops turning, you should be able to figure out why the higher angular velocity won't last: friction will eventually reduce any difference in angular velocities between the fluid and the torus to 0.

Obviously, the earth and atmosphere are not a torus or water, and the actual dynamics of the air is much more complicated. However, this should make clear why the most common answer of: "It spins faster briefly, then slows back down again" is essentially correct.

On the other hand, if you want a more permanent increase, just leave the engine(s) running. You could create a permanent "jet stream" at the level of your engine (much more effective if you have many engines placed around the equator).

Lawnmower Man
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    Your system is unbalanced. You would have to use two propellers diametrically opposite to each other, each adding to the flow. – chasly - supports Monica Jan 19 '21 at 12:14
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    @chasly-supportsMonica - why? what would happen if you just use one propeller – Fattie Jan 19 '21 at 14:19
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    Lawnmower - that is a GREAT thought experiment by the way, bravo – Fattie Jan 19 '21 at 14:19
  • @chasly-supportsMonica Do you mean to cancel the torque from the rotation of the propeller? i.e. to keep the torus from wanting to spin end-over-end? – evildemonic Jan 19 '21 at 15:57
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    @evildemonic - Yes to counteract the torque. Also to centralise the C of G of the whole structure. If you don't do those two things, the motion will be chaotic. – chasly - supports Monica Jan 19 '21 at 16:21
  • @chasly-supportsMonica the point was not to create a perfectly balanced system, but simply to harness physical intuition to illustrate the principles involved. If the propeller is sufficiently small/slow relative to the torus, the secondary rotations will be minimal. Technically, the spinning turbine of the jet engine in the OP's example also exerts a secondary torque on the planet, and this is preserved in my example. – Lawnmower Man Jan 19 '21 at 18:09
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Even if the device is able to eject gas out of the atmosphere (and I don't think such device, if existed, could be considered a jet engine), unless the gas reaches escape velocity in a high enough altitude so air viscosity is negligible, the gas jet will just follow a suborbital trajectory, falling right back into atmosphere, what would eventually cancel out the increase in angular momentum.

A 1 meter wide gas jet at sea level, even at escape velocity, would never make into upper atmosphere. All speed would be dissipated, first as huge sonic boom, and later as turbulence.

lvella
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  • In-between in the velocity range there is orbital speed (gas in orbit). – Peter Mortensen Jan 19 '21 at 09:39
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    @PeterMortensen Orbital speed alone can not put gas in orbit, you also need a proper vector, which can not be achieved if the accelerating device is fixed on a point of the sub-orbital trajectory that intersects with Earth and/or a significant portion of the atmosphere. But you are right, maybe can be done if the device is protruding from the atmosphere, which I didn't consider. – lvella Jan 19 '21 at 11:01
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Addition to other (correct) answers:

Depends on the size of the engine. If it's large and powerful enough that it blasts parts of the atmosphere (and/or its propellant) into space, then you have a net force acting on the planet as a whole. Otherwise, no - as outlined in several other answers.

As for the feasability of such an engine... that's a different question altogether.

Tom
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I think we can say "maybe, but even if it happened, there'd be no way to predict or prove it."

The other answers are correct, assuming a closed system. So can we find a mechanism (however silly) which could open the system, given a single, regular-sized jet engine?

  1. Premise: the butterfly effect means that even a small disruption of air could have a decisive affect on major weather formations, albeit in a chaotic and unpredictable fashion that we could not deliberately exploit.
  2. Premise: major weather formations affect the rotational speed of the planet: greater air density in the upper atmosphere slows the planet; greater density lower down speeds it up. Basic conservation of linear momentum.
  3. Premise: atmospheric escape happens when atoms and molecules in the upper atmosphere reach escape velocity: Earth loses about 3kg/sec this way.
  4. Hypothesis: when upper atmosphere mass is higher, atmospheric escape is greater. This seems a reasonable assumption, but I have no evidence for it.

If the hypothesis is true, then we have joined all the necessary dots: we've a mechanism for the jet to affect the weather; a mechanism for the weather to affect planetary rotation in the short term; and a mechanism for the short term effect to have permanent repercussions.

It'd be impossible to predict or prove what effect the jet had, if any; but it could have an effect.

Dewi Morgan
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Ok, so what will happen is that the rotation of earth will speed up, but negligibly.

On the other hand, the atmosphere will also rotate, but on the opposite direction. So, soon after the propulsion, the friction between the earth and the atmosphere will stop each othe and everything comes to normal.

SK-the-Learner
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    The last sentence is ... may I be blunt? Nonsense. The Earth (ignoring some minor tidal forces) stops only because the atmosphere brakes it, so both necessarily come to a stop (minus their usual movement) over the same period of time. – Peter - Reinstate Monica Jan 18 '21 at 11:13
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A jet engine requires fuel.

Let us build it over a huge oilfield, next to a refinery. Over the years (centuries?) that it runs, the evacuation of the oil deposit will cause the Earth to lose mass and also to shrink slightly in diameter through subsidence. This will tend to speed up the rotation of the Earth.

There is no way to return all the exhaust gases back to the Earth and so the net angular momentum of the solid part of the planet will change permanently.

chasly - supports Monica
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Total angular momentum with respect to the center of mass, in practice the center of Earth, is conserved. The plane will contribute angular momentum to the atmosphere and to Earth, totaling to zero. After a short while the air will give back this angular momentum to Earth and the original value is restored. In the mean time Earth has gained a tiny advance on its rotation, while the atmosphere has gained a tiny lag.

my2cts
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As other answers have stated there would be a small change in the Earth's rotation rate and a change in the opposite direction to the atmosphere's rotation rate. The size of the change would be limited by the increased friction between the earth and the atmosphere, and so the length of a day would settle to a new value if the jet engine were left running long enough. After the jet engine is turned off the increased friction would return the Earth to its usual rotation rate and the length of the day would revert to its natural value.

Peter
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Yes, but it is so minuscule that it wouldn't be noticed. It's the same as if your friend was driving a car and you stick your head out of the window and blew in the opposite direction of the car's movement

Alex
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Total angular momentum with respect to the center of mass, in practice the center of Earth, is conserved. When the airplane takes off, it acquires an angular momentum antiparallel to the Earths rotation axis. The total angular momentum of Earth plus atmosphere increases by the same amount. This angular momentum will eventually distribute over the entire planet. Long before this proces is completed the plane lands or crashes. Assuming it makes no turns its landing will decrease the earth + atmosphere AM back to its original value. If it turns while being up it again imparts angular momentum to the atmosphere. In the end it all adds up to zero but there are transient atmospheric effects.

my2cts
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