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How come a plane flying at constant velocity experiences gravity? If you were in a space capsule flying (not accelerating) you would feel weightless until you hit the ground. Why not a plane?

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    $\begingroup$ Hi Chris K. It's usually a good idea to wait a day or so before accepting an answer on Stack Exchange. Accepting an answer, while it doesn't prevent adding additional answers, signals to the community that you feel your question has been answered satisfactorily, which can result in it receiving less attention from the community than it otherwise would. $\endgroup$ – a CVn Jan 30 '18 at 9:11
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    $\begingroup$ Isn't this a better question for the Physics SE site? $\endgroup$ – Cloud Jan 30 '18 at 12:35
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    $\begingroup$ If you're in a plane and you don't feel gravity, you're in deep trouble. Unless you're on a dedicated microgravity flight. $\endgroup$ – gerrit Jan 30 '18 at 13:28
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    $\begingroup$ obligatory XKCD: what-if.xkcd.com/58. You have to go sideways really fast to continually miss the earth as its gravity pulls you in a circular path (which we call an orbit). $\endgroup$ – Peter Cordes Jan 30 '18 at 13:41
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    $\begingroup$ The surface of the earth at the equator is travelling around the center of the earth at roughly 1000 miles per hour. Why do you feel gravity standing on the equator? $\endgroup$ – Shufflepants Jan 30 '18 at 16:08

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This is the difference between flying and in orbit. In orbit, you are indeed falling toward the earth, but the spacecraft is too, and you're going fast enough that you keep missing the earth.

In an aircraft, because it's staying aloft due to lift, it is not falling. This is why you experience the pull of gravity on a plane.

Some aircraft are designed to feel weightlessness, see Vomit comet.

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    $\begingroup$ @ChrisK: In an aircraft in level flight, you're are still affected by the force of gravity. The aircraft's lift is pushing you the opposite direction. If the sum of all forces is zero, is means you are in equilibrium, meaning you are not being accelerated in any direction; it does not mean that you are "weightless". $\endgroup$ – Greg Hewgill Jan 30 '18 at 7:14
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    $\begingroup$ Actually, one should say that the force, you feel in an aircraft is not a gravity but the floor (seat etc.) pushing against you. You never feel gravity per se, you feel forces on your body which counteract gravity pull. In the orbit, there is still gravity pull on your body, of course, but spacecraft is not pushing against it, so therefore the weigthless feeling. $\endgroup$ – Martin Jan 30 '18 at 8:07
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    $\begingroup$ @ChrisK: You, as a human, would fall to the ground. The plane, however, generates lift that keeps the plane up. Since you are in the plane, the plane stops you from falling. The force the plane enacts on your body to stop you from falling is what you call "feeling gravity". A capsule in orbit, however, has no lift, and the vehicle is in free fall just like you are. Since the vehicle doesn't enact a force on your body you don't "feel" gravity. But make no mistake, even people in orbit are subject to gravity, they just seem to be floating due to their frame of reference with the capsule. $\endgroup$ – Flater Jan 30 '18 at 12:20
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    $\begingroup$ @ChrisK: Think of the vomit comet. When the plane is almost in free fall (and your body is too), it looks like you're almost weightless (relative to the plane, which is your frame of reference). Being in orbit is sort of like being in that freefalling plane, but you never hit the ground because you keep missing it. $\endgroup$ – Flater Jan 30 '18 at 12:22
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    $\begingroup$ @ChrisK "Well if you're not falling and the sum of all forces is zero then wouldn't you still be weightless?" The only way to feel weightless is to be falling. $\endgroup$ – Shufflepants Jan 31 '18 at 15:48
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Why do we feel gravity on a plane?

Exactly for the reasons we feel gravity when traveling on a train:

  • We're not free falling (the cabin floor prevents this to happen).
  • We're not at orbital speed which is about 28,460 km/h.
  • We're not flying very tight curves that could create a free fall (but only for a few seconds anyway).

Gravity and weight

Everything is weighty everywhere in the cosmos as soon as it is subject to some acceleration (e.g. gravity acceleration, but not limited to it) and it tries to oppose this acceleration.

So there are only two means to escape gravity acceleration effects:

  • Remove gravity with another exactly opposite acceleration. This is "the satellite way". The satellite own speed and its circular trajectory create (as viewed from the satellite) a centrifugal acceleration exactly opposite to gravity acceleration. Effects of both accelerations disappear.

  • Remove everything preventing gravity to fully act, this is "the free fall way". Gravity wants us to fall, then we just remove everything preventing us to fall, starting with the floor and/or the ground. When we jump from some height we're in micro-gravity for a short time, and then at the hospital if we underestimated the time. This is also what some aircraft do for 30s to train astronauts ("0G flight"). While the gravity still exists, its effects are cancelled by accelerating with the "gravity flow".

In both cases, the aircraft and the satellite experiences "micro-gravity" (which means a residual gravity in the order of some $\small \mu g$). Any mass subject to micro-gravity is (nearly) weightless.

For the physicists here, there is actually a single case, as a satellite in orbit is also in free fall and there is no centrifugal force, provided we select the appropriate frame of reference for the observer (an inertial frame). If we wanted to be even more rigorous, Einstein also intuited gravity is actually fictitious (if I may say) itself, an idea which led him to the discovery of the general relativity and the space-time curvature

Constant velocity vs constant speed

How come a plane flying at constant velocity experiences gravity?

Micro-gravity never happens in a trajectory at constant velocity.

The reason is because constant velocity is constant speed and also constant direction:

  • Constant speed means we are not free falling, else we would accelerate towards Earth.

  • Constant direction means we are not creating any centrifugal acceleration either, because it requires changing direction.

When satellites are in circular orbit, they are not at constant velocity, they are at constant speed.

Following their orbit, the direction of their displacement is constantly adjusted, hence velocity constantly varies, which allows them to create a centrifugal acceleration exactly opposite to gravity.

Can we create micro-gravity on a plane (or on a train) moving horizontally?

Horizontal doesn't mean "in straight line". It means at right angle from the direction of gravity (the local vertical), so when moving horizontally on large distances, we are actually following Earth curvature.

If the plane/train follows Earth curvature (hence changes direction constantly), we could in theory achieve micro-gravity, but at the condition we travel very fast, a bit faster than the ISS (27,560 km/h at the current time), about 28,460 km/h. In such case we are in orbit at altitude zero (orbital trajectory doesn't depends on altitude).

This is not possible in practical, an enormous amount of power would be required and everything would melt due to friction.

Micro-gravity in a plane flying a specific curve

But as explained in Can one fly up side down while a glass of water keeps full due to g-forces?, we can create micro-gravity by flying a specific trajectory. In that case, the speed we are missing is replaced by constant changes in direction along the curve. This gives nice videos, like the funny weightless dog with the two unperturbed guys:

enter image description here
Source


To sum up

Weightlessness is the consequence of being subject to micro-gravity which can be obtained:

  • At constant speed we need to follow a curve which creates an acceleration exactly opposite to gravity. This either requires moving at a large and specific speed (orbital speed) or doing relatively tight turns at limited speed.

  • In free fall we must follow the downwards trajectory and permanent acceleration dictated by gravity, which means, e.g. in 35 seconds, and 6 km lower, we are already moving hypersonic! Not so comfortable, and that's only for the first 35 seconds!

For feasible and durable micro-gravity at low altitude, the two techniques must be combined.

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    $\begingroup$ "Orbit doesn't mean absence of gravity" It's even more brutal than that; orbits are not possible absent gravity. When you are in an orbit, you are following a curvature formed by the interaction of velocity and a gravitational field. This is why you end up with seemingly odd results like spacecraft travelling in a straight line, but not in the reference frame you're used to. $\endgroup$ – a CVn Jan 30 '18 at 9:08
  • $\begingroup$ This should be the accepted answer IMO $\endgroup$ – Travis Bear Jan 30 '18 at 22:04
  • $\begingroup$ Satellites are also free-falling. They do not create new forces. $\endgroup$ – immibis Jan 31 '18 at 2:12
  • $\begingroup$ @MichaelKjörling - Well, an unpowered orbit is not possible absent gravity (or some other force, like magnetism). :-) $\endgroup$ – T.J. Crowder Jan 31 '18 at 10:22
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    $\begingroup$ @T.J.Crowder If it's powered, I'd argue that it's not an orbit; rather, that's being on a powered trajectory with continuous or non-continuous trajectory changes, with the trajectory possibly forming a closed ellipse. (Not saying that doesn't have its uses; look at Rosetta's trajectory around comet 67P.) Good point though on "some other force", but since in our universe gravity is pretty much ubiquitous, well... $\endgroup$ – a CVn Jan 31 '18 at 10:25
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You never actually “feel gravity” at all, not in orbit, not on a plane and not on solid ground either.

What you do feel on ground is the earth pushing against your feet, with a force that exactly cancels out the gravitational acceleration. As soon as you stop that force, e.g. by cutting the ropes in an elevator, the gravitational acceleration would very quickly change your velocity, downwards, which of course inevitably brings you back to the ground (where it will hurtfully reaffirm its upward force...) in situations like an elevator. We're completely used to that upwards force as the normal state, so much that we don't even notice it as a force and instead talk about the “gravitational down force”, but physically that's not really the force that's there.

In a plane, the situation is much the same: the force you're feeling is the force of air flowing around the wings, pushing the entire plane upwards. Without that force, the plane quickly stops travelling at constant velocity and instead travels ever faster towards the ground.

Now, for a space capsule in orbit, this actually happens as well: here, there isn't any force counteracting the gravitational acceleration, so it is in free fall. But because it has a blisteringly fast horizontal velocity, there's not enough time for it to fall down onto the ground – it “misses the Earth” instead, and thus continues its orbit.


The only place where you could actually feel gravity itself is close to a black hole, where your body would be getting stretched out by the tidal forces... but that never happens in homogeneous gravity field, and any sufficiently large/distant field is approximately homogeneous.

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    $\begingroup$ Spaghettification is still one of the best terms to come out of modern astronomy! $\endgroup$ – Cort Ammon Jan 30 '18 at 16:53
  • $\begingroup$ Not entirely true. The inner ear can sense which way is up, also called the sense of balance. Still, whether we experience it or not, the question still seems to be about a real force that exists. $\endgroup$ – Octopus Jan 30 '18 at 20:18
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    $\begingroup$ @Octopus No, neither your inner ear can sense gravity. It just feels from which side are floating particles in the ear supported by the ear channel, which is, in turn supported by your body, which, in turn by the floor or seat of an airplane. All your "sensing" is nothing more than strain/deformation of some of your tissue. Homogeneous force field only does not cause any deformation. $\endgroup$ – Martin Jan 30 '18 at 22:31
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    $\begingroup$ @Martin, your argument is completely semantic. The OP has already pointed out that a plane passenger feels something and a space capsule pilot does not. Call it not gravity if you like, but there is a sensation! $\endgroup$ – Octopus Jan 30 '18 at 23:43
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    $\begingroup$ @Octopus It can be seen as semantic one, but I believe, it is better way to think about situation. If you imagine completely empty space (so totally no gravity), only with your chair (and some capsule or so, so you do not suffocate :) ) which will exert exactly the same force on your body like when you are sitting at your desk at home, you will get exactly the same feeling of "gravity" despite there is no real gravity. (yes, you can state that it is because gravity/acceleration equivalence, and you will be right, still inner ear is no way special part of your body here). $\endgroup$ – Martin Jan 31 '18 at 8:48
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We are, the speed is not sufficient for this to be easily observable. We are actually a little bit lighter while in a plane, because it is also circling the Earth, just like a spaceship does, but even for SR-71 at top speed (assuming 3540 km/h = 983 m/s) the effect is too small to be sensible:

$$ g = \frac{V^2}{R} = \frac{(983\frac{m}{s^2})^2}{6400000 \ m} = 0.15 \frac{m}{s^2} = 0.015 \ g $$

(g is close to 9.8 on Earth). It is even not really minor, but I doubt if 0.015 g acceleration is very observable. For Boeing 747 (assuming 988 km/h) this is only 0.0011 g.

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    $\begingroup$ Of all the answers, this is the only one which points out that there is a smooth shift from feeling gravity (i.e. 1g) to feeling weightless (i.e. 0g), and provides the equation that you can use to calculate how much acceleration you feel. In fact, this effect can play an important role once you go faster than the SR-71, into the realm of hypersonic aircraft. $\endgroup$ – Cort Ammon Jan 30 '18 at 16:59
  • $\begingroup$ So with a 747, we are about 1% lighter? Who-hoo! $\endgroup$ – chux Jan 30 '18 at 18:03
  • $\begingroup$ @chux If your 747 is accelerating straight downwards at 1% of gravity. $\endgroup$ – immibis Jan 31 '18 at 21:49
  • $\begingroup$ @immibis Who-Noooo! $\endgroup$ – chux Jan 31 '18 at 21:58
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It is not a gravity, you feel, but the floor (seat etc.) pushing against you. You never feel gravity per se, you feel forces on your body which counteract gravity pull. In the flying airplane these forces come from wings' lift, but in the orbit, there is no such counterforce, so you feel weightless despite gravity is till there.

All the feeling of weight comes to your brain from various strain or deformation sensors in your tissues. So in order to feel weight, there has to be force deforming your body. Homogeneous gravity field (gravity around Earth is homogeneous enough for these purposes) exert exactly the same force on each single point in your body, therefore causing no deformation.

On the other hand, floor, seat etc. supports your body locally only and the force needs to be "distributed" through your body, which causes strain in the tissues and "feeling of weight".

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  • $\begingroup$ This captures what I was going to write into an answer: the concept that we call "weightlessness" is really an internal property: it is the absence of these distributed forces. It actually doesn't matter at all whether they are caused by true weight (i.e. the effect of gravity pulling each part of the body down), or if it is caused by another effect (such as centripetal acceleration in many amusement park rides). $\endgroup$ – Cort Ammon Jan 31 '18 at 22:24
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An aeroplane flying does not fly fast enough to become weightless. A person inside an aircraft flying at constant altitude becomes weightless if the centrifugal$^1$ force F$_C$ = $( m \frac{V^2}{R})$ from following the curvature of the earth equals the force from gravity ($m \cdot g$).

enter image description here

$$ m \cdot \frac{V^2}{R} = m \cdot g \Rightarrow V = \sqrt {R \cdot g}$$

With g = 9.81 m/s$^2$ and R = 6,400 km even at cruise altitude, we get V = $\sqrt{9.81 \cdot 6.4 \cdot 10^6}$ = 80,000 m/s. At that speed 10 km above the earth's surface, you'll be weightless

$^1$ makes for an easier to understand graphic.

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    $\begingroup$ "An aeroplane flying does not fly fast enough to become weightless." Well, not in level flight. It's perfectly possible to experience 0 normal force in accelerated flight. This will not be the most comfortable experience for the passengers, though. $\endgroup$ – reirab Jan 30 '18 at 16:49
  • $\begingroup$ An excellent and perfect answer. $\endgroup$ – Fattie Jan 30 '18 at 18:00
  • $\begingroup$ @reirab - something to consider. If the plane is simply falling (the vomit comet), (you and I understand that .....) that's a totally different "junior physics popular science topic" to understand, than the worthwhile "junior physics popular science topic "why astronauts in orbit feel weightless"..... it's a shame to conflate those two (interesting but different) "junior physics popular science topics" - IMO ! $\endgroup$ – Fattie Jan 30 '18 at 18:10
  • $\begingroup$ FWIW koyovis - "An aeroplane flying does not .. become weightless" Actually IMO an aircraft in normal level flight is, in fact, precisely weightless. Using the usual "weight V. mass" ! high school definition of those two terms. Note that in it's own frame of reference, if you hung the 747 by a scale - it would weight zero. (You would have to, obviously, have another chopper or something pacing it, just underneath, to weigh it using a large bathroom scale, to be in it's frame of reference. Of course, it would weigh zero. As would (say) a balloon-type craft at vertical rest.) $\endgroup$ – Fattie Jan 30 '18 at 18:13
  • $\begingroup$ @Fattie Orbit is 'simply falling.' The reason for feeling weightlessness is the same in both cases - namely, there's no normal force pushing up on you to oppose the force of gravity pulling you down. That is, you're in freefall (accelerating somewhere between 0.9 and 1.0 g toward the center of Earth) in both cases. The only difference between them is that the orbital craft is also moving sideways fast enough to continually fall around the Earth, whereas the airplane isn't and would eventually experience a very abrupt upward force exerted by the ground if allowed to continue its fall. $\endgroup$ – reirab Jan 30 '18 at 18:50
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Consider the "vomit comet" flights, where they intentionally fly the same plunging path that a bowling ball would "fly" if gravity took it. If you simply continued that flight path, you would go SPLAT.

Wings are a funny shape, specifically to create lift. That is so they can create a flight path other than that one.

The "gravity" you feel on the plane is the wings doing their thing. The wings themselves have been adjusted to counteract gravity exactly, so the force feels the same as gravity. The reason to counteract gravity exactly is to remain at the same altitude, the one ATC assigned them so they don't hit other aircraft...

... Or (this is a little more complicated) remain at a constant rate of climb/descent for passenger comfort and simplicity. If you are moving at a constant speed, acceleration is zero, and gravity is an acceleration effect.

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The answer to this question could not be simpler.

At a given altitude, you need to be going a certain speed, to achieve "weightlessness".

At sea level, that speed is 28,500 mph.

At 30,000 feet, that speed is 28,400 mph.

In an airplane, you are not going fast enough.

That's all there is to it.

You are not going fast enough.

(Note that exactly the same could be said of a train or car. If you were going 28,500 mph in a TGV, you would get the "weightless" effect. You would float around, etc, inside the TGV exactly as astronauts float around inside a space station.)

Use this calculator, to know the speed you need for different altitudes:

http://www.calctool.org/CALC/phys/astronomy/earth_orbit

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XKCD covered this in detail. While the other answers cover all the physics angles, XKCD does a great job of bringing the science down to an easy-to-understand level (emphasis mine)

Gravity in low Earth orbit is almost as strong as gravity on the surface. The Space Station hasn't escaped Earth's gravity at all; it's experiencing about 90% the pull that we feel on the surface.

To avoid falling back into the atmosphere, you have to go sideways really, really fast.

The speed you need to stay in orbit is about 8 kilometers per second. Only a fraction of a rocket's energy is used to lift up out of the atmosphere; the vast majority of it is used to gain orbital (sideways) speed.

This leads us to the central problem of getting into orbit: Reaching orbital speed takes much more fuel than reaching orbital height. Getting a ship up to 8 km/s takes a lot of booster rockets. Reaching orbital speed is hard enough; reaching to orbital speed while carrying enough fuel to slow back down would be completely impractical.

The fastest aircraft ever, the X15, couldn't stay in orbit because it only traveled at about 2km/s, or 25% of the necessary speed.

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Newton's Cannonball.

https://en.wikipedia.org/wiki/Newton%27s_cannonball

In a plane, you're the cannonball marked "A", and the only thing stopping you from plummeting straight into the ground is the lift generated by the wings.

enter image description here

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