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(I am not quite sure whether I should be asking this question here or in Physics exchange or biology exchange, but i decided to place it here due to the context that this question came to my mind.)

It's my understanding to when a "point" sized object is placed between two sources of G force(like two stars for example), The effect of those G forces will, for my lack of coming up with a better term, start cancel each other out. As best explained in This question from Physics exchange.

But of course no object in real life is really point sized, From what i grasped from yet another question in Physics exchange which dealt with what will happen if an object is caught between two black holes as their event horizons are merging(sadly i can no longer find that question.), I assume a normal "object" will simply get torn apart as if been pulled by ropes from both sides when opposing gravities become powerful enough. But i assume that's what will happen at very high G forces.

I am wondering how will a human will feel and how his or her body will be affected when affected by 2 opposing G forces. I got this question in my mind first time during an episode of Mayday TV series in which the during an incident, Aircraft continued speeding toward ground while the engine was still running full power(ish) placing human body between two opposing G forces. Of course in that particular incident the aircraft in question was a turboprop, and the whole incident took only few seconds and pilot has been considered unconscious at that time. Also I imagine earth's gravity has completely overpowered any effect that g force of a turbo prop engine might produce in that time span.

Now to my question, Hypothetically let's say a human is in a spacecraft moving toward earth with engines running generating G-force equal to 4m/s2, at the height of 100k kilometers above center of earth (where according to Wikipedia the earth's g-force should be around 4m/s2), How is the best way to describe how that human will feel? And also if the spacecraft's thrust and altitude for some reason do not change, How will the opposing G forces effect that human's body and health over time.

P.S: Yes i am fully aware that if a spacecraft is running it's engine while moving toward earth, then most likely something is seriously wrong :)

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    $\begingroup$ Spoiler alert! If you don't recognize the reference better read Larry Niven's short story Neutron Star before reading anything there! scifi.stackexchange.com/search?q=general+products+hull $\endgroup$
    – uhoh
    Feb 10 '20 at 11:39
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    $\begingroup$ Side note: a spacecraft running its engine while moving TO Earth is not always wrong. Apollo-4 test flight did it in 1967, as well as Orion EFT-1 test in 2014. In both cases the aim was to increase atmospheric reentry speed. Higher speed was needed for testing of spaceship's heatshield at velocities about 11 km/s. $\endgroup$
    – Heopps
    Feb 10 '20 at 13:26
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    $\begingroup$ I instantly flashed on Neutron Star as well. $\endgroup$ Feb 10 '20 at 13:29
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    $\begingroup$ Since G-force is just a force, consider the Medieval age practice of tying a person to two teams of horses and setting them off in opposite directions. Same here. $\endgroup$ Feb 10 '20 at 14:03
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    $\begingroup$ @CarlWitthoft, not the same. Forces pulling opposite directions on two different locations of a body, vs. one force pulling on EVERY particle of the body with another pushing on the body hard enough to accelerate it toward the other force. $\endgroup$
    – WGroleau
    Feb 11 '20 at 0:41
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If you imagine that in your example the rocket's engine is not initially firing. The rocket and its occupants are falling freely towards the planet below. The occupants will feel weightless, because there is no normal force from any direction.

Now, if you point your rocket's nose down and start the engines, the occupants will fall towards the bottom (or back, if you like) of the rocket, and will feel a normal force from the floor underneath them. They'll feel as if they're in a perfectly normal .4-ish g gravity field. They'll probably feel as if the planet they're now travelling very quickly towards is above them. The two accelerations don't "cancel out", as they're both acting to accelerate the rocket towards the centre of the planet.

As the rocket gets close to the planet, acceleration due to gravity will increase. If the engine wasn't running, this would be imperceptible, because the rocket would still be freely falling. If the engine was still running, they'd still feel a 4m/s2 acceleration towards the tail of the rocket, right up to the point where they hit thick enough atmosphere to develop aerodynamic drag, at which point we leave the bounds of your question.


What you're probably most interested in, and seemingly reaching towards, is when the acceleration due to gravity is different at one end of a person than at the other, producing a tidal force. It is difficult to produce a perceptible tidal force on the scale of a human in a gravity well like Earth's... the gradient just isn't steep enough, and people just aren't long enough.

Tidal forces around Earth can and do have effects on human-made objects though, and you can take advantage of (or suffer from) this if your spacecraft is long enough. This is used to orient satellites via gravity-gradient stablisation, for example.

For a suitably large, or suitably delicate body, there's the notion of the Roche limit, the point at which the strength of tidal forces exceeds its tensile strength and the object will disintegrate. This happened to Shoemaker-Levy 9 following a close encounter with Jupiter. For particularly strong gravitational fields, such as those around a neutron star or black hole, this effect is referred to as spaghettification. It would obviously be a bad thing to happen to a human.

Whilst there would undoubtedly be side effects to spending any time in a gravitational gradient steep enough to affect humans, the additional hazards imposed by the stellar remnant that generated that gradient would probably incinerate or fatally irradiate any humans foolish enough to get that close.

You might consider a centrifuge to be one model for the effects of tidal forces, though I suspect that coriolis effects will overshadow the things you'd be most interested in. On most centrifuges built for use on humans, the radius is small enough that there's a non-trivial difference in centrifugal force on the scale of a human.

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    $\begingroup$ Spaghettification may be the most self-descriptive phrase in all of space exploration, other than perhaps Rapid Unexpected Deconstruction. $\endgroup$
    – Cort Ammon
    Feb 11 '20 at 3:45
  • $\begingroup$ Great answer! But is the 4 meters per squared some new notation that I haven't seen before, or are you missing an s? $\endgroup$
    – BThompson
    Feb 11 '20 at 14:20
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    $\begingroup$ @BThompson there was indeed a missing s. $\endgroup$ Feb 11 '20 at 14:22
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You can actually experience it yourself.

Take for instance an ice-skating athlete, when they perform one of those spinning numbers: the g force in one hand is minus the g force in the opposite hand. And it gradually varies through all their body.

What will happen to the human body? With a g force strong enough, it'll get stretched and torn apart.

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As long as you are in freefall, you don't feel the acceleration of gravity.

The reason is that you can't feel force directly, you can only feel force when it doesn't act uniformly on all parts of your body.

Example 1: Standing on the ground. Gravity acts uniformly on all parts of your body, but the normal force from the ground acts only on your feet. You can feel this difference.

Example 2: A cosmonaut in Earth orbit. Here, the force of gravity acts equally on all parts of the cosmonaut's body, with no other force acting. Hence, they experience weightlessness.

Example 3: Your example.

Gravity acts uniformly on the human, so they don't feel it. But the spaceship's floor is pushing at them at 4m/s^2, so they can feel the difference between where the floor pushes, and where it doesn't. (and even if you where inside a tank of water, it would only push on your skin, not on the inside too like gravity does).

In fact, they would feel the exact same inside an accelerating spaceship, as they would by feeling gravity standing on a solid platform. This is a very deep connection that for instance relativity deals with.

There are cases where gravity doesn't act quite uniformly on you. For instance, your feet are a little bit closer to the centre of the Earth than your head, so there's a tiny difference. These are often called tidal forces. At 100k km (100 mega meters?), this difference would be even smaller than usual.

But if the size of the body is greater, the tidal forces get greater too. The "feet" and "head" of a moon are very far apart, so if they are too close to their planet, gravity can break them apart (Saturn has pretty rings for this reason)

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  • $\begingroup$ This is not exactly what the person asked. If you were half-way between two massive objects & each was exerting 10Gs on you, you'd be held in place but feeling extreme pain as your body's every molecule felt the "tidal" force. $\endgroup$ Feb 10 '20 at 14:06
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    $\begingroup$ @CarlWitthoft Certainly not, as you have left out the gradient part, which is what causes tidal forces. For there to be a strong gradient at only 10Gs, you would have to be within a couple of hundred meters of the masses. A planet won't fit, and a black hole would have a much higher acceleration. $\endgroup$ Feb 10 '20 at 21:22

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