Q: If gravity suddenly increased would airplanes fall out of the sky, or would it compress the air in such a way that airplanes could keep flying?

The original question was: “In Kurt Vonnegut’s “Slapstick” there is talk of Earth’s gravity randomly changing, causing planes to fall out of the sky. Since planes fly because of air pressure variance between the top and bottom of the wings, would an increase in gravity really make them fall? The weight of air would increase by the same relative amount as the weight of the plane.”

Most plausible-ish answer so far: If gravity suddenly increased, yes, planes would fall out of the sky. When a plane is maintaining a constant altitude, the force of gravity and the force of lift are equal. If gravity were suddenly increased, there would no longer be an equilibrium, and the plane would drop. The plane would have to pick up speed to increase the lift to match the new level of gravity. Although air pressure does affect lift (the less air pressure, the faster you have to go to get the same lift), in this case I think the increase in air pressure and the increase in the weight of the plane would cancel each other out.

Physicist: They kinda do cancel out!

So, once gravity has been turned up, and the atmosphere has had a chance to equilibrate at a higher pressure and density (and get squished shorter), planes will still be able to fly (in theory).  But if, for example, the gravity doubles, the planes will need to fly through double-density air to stay up (or fly about 41% faster).  But in so doing, they’d experience double the drag.

Unlike airplanes, dirigibles and hot air balloons would do just fine.

Lift, L, is given by \frac{1}{2} \rho v^2 A C_L, where \rho is the air density, v is the velocity, and A and CL are constants that depend on the air craft.  When a plane is flying it needs its Lift force to be greater than its weight.  You may notice that your computer (or phone, or tablet, or whatever) isn’t flying right now.  At best it’s falling.  That’s because its lift force is zero.

Drag, which affects planes, cars, sleighs, or anything else that moves through air is given by \frac{1}{2} \rho v^2 A C_D (this equation applies well to things that are faster than snails, but slower than sound).

That being said, clearly planes can fly in air of different densities.  The difference in densities between “cruising altitude” and sea level is about a factor of 4.

The problem is that, generally, the Drag and the Lift are proportional.  You may have already noticed that the equations for Lift and Drag are practically the same: L=\frac{1}{2}\rho v^2AC_L and D=\frac{1}{2}\rho v^2AC_D.  CL and CD change a bit for different scenarios, but aside from that, for any particular aircraft, the equations are proportional.

So, if the gravity doubles, then the lift needs to double (because the plane weighs twice as much).  The plane can do this by flying faster, or flying through denser air.  But whichever method is used to double the lift, will also double the drag.  The long and the short of it is: if gravity increases by some amount, then the amount of power required to keep the aircraft aloft will increase by the same amount.  For example, 5 times gravity would require 5 times the power.

So while airplanes can fly in higher gravity, it may take more power than their engines can output to keep them in the air.

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30 Responses to Q: If gravity suddenly increased would airplanes fall out of the sky, or would it compress the air in such a way that airplanes could keep flying?

  1. Moritz says:

    It’s even worse, since the propulsive efficency of the jet engine will probably decrease when flying at a slower speed with higher air pressures.

    And I really can’t imagine “Top Gun” at half speed.

  2. The Physicist The Physicist says:

    The volleyball scene would be even more weird and out of place.

  3. Mark says:

    Didn’t Einstein prove that there is no such force as gravity?

    If so, Why do physicists keep talking as if there is a force of gravity?

  4. The Physicist The Physicist says:

    What Einstein did proposed (and was later experimentally verified) is that the “force of gravity” is really a “pseudo-force” in the same way that the centrifugal force or coriolis force is a pseudo-force. That is, it only seems as though an object is experiencing a force from some perspectives.
    That being said, you really have to do some mental acrobatics (in a curved four dimensional Minkowski space) to keep in mind the perspective from which gravity isn’t a force. Much easier to just say “pseudo-forces are forces 90% of the time, and if there’s a problem we’ll sort it out”.
    In this case, there’s no problem with saying that gravity is a real force.

  5. Ed Matzenik says:

    I think you will find that NASA scientists have proved that planes do not fly because of air pressure variance between the top and bottom of the wings, they fly because of momentum transfer from the air that the wings deflect downward.

  6. The Physicist The Physicist says:

    You’re right. But, “momentum transfer” is just a more precise way of saying “force for some amount of time”. In this case that force takes the form of a difference of pressures over the wing’s area.

  7. I am doing a research on why do airplane do not fall when they are flying,why they do not consider a force of gravity that pulls something down the earth?

  8. what is the purpose of the black box in the airoplane

  9. Consolata says:

    and what about those satellites?

  10. The Physicist The Physicist says:

    Airplanes are affected by gravity, just like everything else, but they fight against it. A helicopter works in a much more obvious way (pushing air down), but airplanes do essentially the same thing. The black box is a very tough recording device that tells engineers what was happening right before a plane crash, so that they can avoid similar mistakes in the future.

    Satellites also experience gravity, but unlike airplanes they don’t fight gravity, they just fall continuously. In fact, there’s a post that talks about that!

  11. Jason says:

    I’m wondering: what would an increase of gravity to 1.5g do to air pressure and how would this affect terminal velocity?

  12. The Physicist The Physicist says:

    The air pressure would be increased by a factor of exactly 1.5. Air pressure is dictated by the weight of the air above you, and since the amount of mass stays the same, increasing gravity just increases the weight.
    Very weirdly, terminal velocity should stay about the same for most objects. The effect of increased weight and increased air density cancel each other out (the equation for calculating terminal velocity can be found here).

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  14. Bob says:

    What happens if a satellite is in orbit at about the altitude of the ISS and then the gravity doubles? Would it fall out of orbit from the increased gravity (assuming it remained at the same speed)?

    In orbit, there is no air resistance and no drag. In this situation, how would a satellite’s behavior differ from an airplane’s?

  15. Ajay Yadav says:

    Is there any way/method to increase gravity except the usual centrifugal force???
    Or any substance can increase gravity??
    & also can magnet attract a human like metals if there is a lot of magnet (worth earth’s core) ??
    Is MASS=Gravity?
    or gravity is something else?
    I’m pretty much certain that gravitational force a formed due to a substance is it so??

  16. Chris says:

    @ Bob: If gravity suddenly increased, a satellite would be pulled towards the earth into a tighter, faster orbit. Depending on how low it already was, and how much gravity had increased, it might be pulled into the outer atmosphere and begin to drag, at which point it would burn and fall to earth. Farther satellites might be able to maintain orbit. The ISS would have to burn in a pro grade trajectory to stabilize and trim their orbit. It’s also worth mentioning that plenty of debris would be falling towards earth, which could result in some satellites being destroyed. The moon would probably get a little closer too (technically a satellite), which means that the tides would become more intense.

    Fortunately this is an impossible scenario. @Ajav: yes, gravity is a function of mass. Even your own body has an extremely slight gravitational force. It takes a lot of mass (like the size of our planet) to generate the level of gravity we feel. The only way to increase the gravity of Earth would be to increase it’s mass.

  17. Marc Tiltman says:

    If gravity were to let’s say, suddenly double, the one very, very important reason why the plane will NEVER actually be able to take off is the strength of the materials it’s made from.
    The materials would of course all be twice as heavy, but of the same strength!
    The plane could become seriously damaged under its own weight. Add to that; fuel, luggage, freight, and of course the hundreds of passengers, all of which twice as heavy. The only place that plane will be going to is nowhere!
    If for this exercise we forget the inconvenient problem of the plane becoming a pile of junk outside departures, it could still fly, theoretically. Also the pilot and cabin crew would also have to for some reason totally ignore the fact they have all suddenly doubled in weight, all movement now very hard work and still attempt a take off.

    Drag may not be a problem, if air is twice as dense; the plane is twice as heavy.
    Weight may not be a problem, the plane is twice as heavy, but the air is twice as dense, so lift would be proportional.

    Obviously the plane will not be able to reach such great altitudes, but should still fly, and with using the same power output.
    The engines would also receive more oxygen to burn the fuel. With air twice as dense before engine compression the same engines could produce more power, provided the engines could withstand producing it.
    The engines starting system would need to be more powerful to get the engines going due to this increased internal compression.

    If gravity were to suddenly double while the planes at altitude, the sudden compression of the air would cause the twice as heavy plane to fall many thousands of feet, its momentum taking it out of control.

  18. Amanda says:

    I have a question about the opposite, what happens if gravity one day fails, stops working at all? All as we ignore the why, only what happens and how fast after gravity stops. In what order would things happen, the atmosphere disappearing, temperatures changing, things starting to float off?

  19. Marc Tiltman says:

    Hi Amanda,
    The short answer is that the effects we would experience of gravity suffering a total failure would be instantaneous.

    The Earth is a sphere because gravity pulls all its mass into this simple very basic geometrical shape. The Earth is also slightly oblate, flattened slightly at the poles, wider at the equator. This is because the Earth is rotating and centrifugal force, the same force we feel on roundabouts is causing the Earth to slightly bulge at the equator to this shape.

    Now take gravity out of the equation and the Earth through centrifugal force will move towards flattening out to become a disc. The outer edges of this new disc shaped Earth would then fly out into space and us with it. This would give the Earth a spiral shape as all its matter gets flung outwards.
    This effect would be very similar to pouring liquid onto a spinning disc, the Earth would disintegrate.

    A sudden total failure of gravity would have us instantly gasping for breath in what would be a vacuum. Gravity of course gives our atmosphere its pressure, take that away, it will suddenly decompress. This decompression would be sudden and totally catastrophic for us.
    However before anyone has the chance to die of asphyxiation, the sudden lack of atmospheric pressure would cause bodily fluids like blood to boil resulting in death. Water boils at room temperature in a vacuum chamber. Fluids in your body contain gasses like Oxygen, Carbon dioxide, and Nitrogen, once in a vacuum those closest to your skin first, would begin to expand into bubbles, resulting in them boiling. You would in effect be in space without a spacesuit.
    Without atmospheric pressure it would also become very, very cold. However, as the Earth breaks up, its hot liquid and semi liquid interior would mix with what was the crust as the planet flattens and sprays outwards. But I’m sure we’d all be dead by this time.
    At the very centre of the Earth it is believed that there lies a vast solid iron crystal which is as hot as the sun’s surface. This iron would otherwise be a fluid if it were not for the extreme gravitational pressure exerted upon it from the Earth’s upper layers. Now without gravity, and the Earth disintegrating, this iron can become fluid and also become flung outwards into space.

    You would think, meanwhile as the sun which is also rotating it would behave in a similar way to the Earth.
    However, inside the sun is equilibrium between gravity and thermonuclear fusion, there’s the equivalent of about ten trillion nuclear bombs going off inside the suns core per second. Gravity is able to equal this pressure giving us our nice stable sun.
    Thermonuclear fusion in stars is primarily driven by gravity, take that away and this also ceases. But although taking away gravity stops thermonuclear fusion, there still exists all the heat pressure which is in the sun at any given moment; the sun rapidly expands, explodes if you prefer.
    If we could somehow survive the Earth disintegrating and watch the sun, we of course would not notice anything different about it for about 8 minutes due to light speed. After that you would be able to watch the sun initially balloon in size, and then gradually flatten out due to its rotation and centrifugal force.
    The sun would continue to expand but at the same time flatten out into a spiral which would also engulf the disintegrating Earth.

    So if you were to survive asphyxiation, blood boiling, and the extreme cold, then extreme heat and extreme cold again as the Earth disintegrates, you would then be cremated as the sun engulfs you.

    The other planets, in particular Jupiter with its already flattened shape due to centrifugal force would be very interesting to watch, if you could.
    You would be able to see it simultaneously expand in size and flatten out into a spiral as its mostly gaseous composition decompresses and becomes flung outwards again by centrifugal force. Jupiter rotates at a far greater rate than Earth, and is of course far more massive.

    Watching the stars, you would see no change at all, whatsoever……..for a very, very long time, light speed of course. But if you could survive this disaster and watch for many, many years you would see over a course of thousands of years the naked eye stars one by one succumbing to the same fate as the sun.
    The closest first of course, you would see one here and there every so often becoming very bright all of a sudden. Depending on which plane of rotation is facing us you may be able to observe some stars flattening out, becoming a line of light not a point, before fading away. Others if we’re viewing them from above the plane of rotation would appear to us as first becoming brighter, then expanding until eventually fading out of sight. You would also see them change colour from white to yellow, to orange, becoming increasingly red just before vanishing. As these stars expand, they will cool as their heat is dispersed out into space.

    What is actually happening of course is that all the stars are succumbing to the lack of gravity simultaneously. Light from stars taking so many years to travel to us makes it seem that they are all still there.
    At first the universe would look no different to how it looks today, but all an observer would be viewing is the light from non existent stars and galaxies.
    When you look out at the night sky, some of the stars you can see, may not actually be there, they could have reached the end of their life cycle long before you were born i.e. the red giant Betelgeuse.

    Galaxies are held together by gravity, these too would disintegrate.
    Gravity is quite possibly the most important force in the universe; it is thought to have been the first to have arrived after the Big Bang. It is the reason why stars form, it is also what ignites thermonuclear fusion in these stars. Stars create all the elements heavier than Hydrogen like Carbon, Oxygen, Silicone and all the metals which eventually through gravity of course form into planets with which life may flourish. Gravity brings matter together.

    If gravity were to suddenly fail the universe would become lifeless with bacteria being the last to be frozen out of existence, and even this would not take very long.

    I think we should keep gravity; it appears to be extremely useful.

  20. Hitesh Sharma says:

    Just a curiously. . What if an infinite amount og mass is supplied to earth.. what will happen after a years ? A next big bang ? Does it mean gravity is nothing but the property of big bang or an matter ?

  21. ricardo says:

    Is sky has a lesser gravit?

  22. blessing fanami says:

    the wiegth of the air would increase by the same relative amount as the wieght of the plane

  23. david anyalewechi says:

    i dont still understand the aspect that the plane hangs on the air

  24. Jane Vincent says:

    Is there any way, while an airplane is flying, to transfer the effects of gravity from the plane to the air around it? Would this enable the plane to fly faster?

  25. sivakumar says:

    What happens if we keep on adding huge amount of weights at a single point on earth and what happens at that point that earth cant hold that weight at that particular point? I’m sorry if my question is a crazy one.. But please some one explain this..tya

  26. nixon says:

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  27. Jett&Nixon says:

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  28. Sandy Zhao says:

    Hi Nixen and Jett this is Sandy, this really helped me too!

  29. Sandy Zhao says:

    What will happen to planes if gravity decrease?

  30. Serena says:

    This is intresting.

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