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I've watched The Truth About Gravity With Professor Jim Al-Khalili | Gravity And Me | Spark where astronaut Chris Hadfield says at 3:55:

To come back to Earth is violent

Then after several seconds of music and video of descent

it can be five times the force of gravity...for quite a long time

I got immediately puzzled as in free fall objects are not subjected to "gravity force": the rest of the video itself explains that.

Just to be sure I've web-searched and found e.g. Return to Earth: An Astronaut's View of Coming Home but more gravity is mentioned when talking about ascent to space.

I've tried to read wiki Gravity turn where some mechanics of descent are explained but have not found about high acceleration on descent.

I consider Jim Al-Khalili a respectable scientist. Looks to me the issue is a video editor mistake overlooked by reviewers. What other explanation can there be?

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  • $\begingroup$ The "force of gravity" is a "G force" -- a force that wants to cause 9.8m/s/s of acceleration. Five times the force of gravity is 5 Gs. When a reentry capsule "hits" the atmosphere it is slowed rapidly, creating large G forces. $\endgroup$
    – Hot Licks
    Nov 27, 2020 at 14:57

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Reentry speeds are fast. Astonishingly fast. The shuttle reentered at 7.8km/s. Now note the units. That's "per second." That's 28,158km/hr. And you have roughly 100 vertical kilometers to do that braking in. Yes, the braking gets to be done at a very shallow angle, which means you have more linear distance to break than the 100km would suggest, but its still a very short time to lose a ton of speed!

This requires a pretty substantial braking force. The force they are referring to is the aerodynamic forces felt by the airframe as it starts to bite into the ever thickening atmosphere. The steeper one's reentry, the more the force has to be.

This force causes a deceleration, of course. And it is not easy for humans to come to grasp with 50m/s^2. It just isn't a concept we have a good intuitive sense of. So what we tend to do is phrase it in terms of gravitational accelerations, dividing out 9.8m/s^2. We can intuitively grasp the idea of feeling like you weigh five times as much as you do when you are standing upright.

We endure these brutal reentry forces, of course, because there's a balance to be played. It would be possible to reenter slower by taking a more shallow angle. However, it can be tricky to control in this environment, and you run into additional heating problems because you spent more time at temperature as you slowly dropped your speed.

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    $\begingroup$ I see, that is slowing speed that is orthogonal to direction toward Earth (orbital rotation speeds)! that is why they feel it with their backs. That is similar to child seats in cars where child' back is facing frond of the car - for same reason - most cars usually decelerate faster than accelerate. $\endgroup$ Nov 25, 2020 at 7:19
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    $\begingroup$ @Martian2020, Not sure exactly what you are saying there, but the couches in a "space capsule" usually are oriented such that on the ascent, the astronauts feel the thrust of the engines pushing the couches against their backs. Then, when it's time to come home, they turn the capsule around and re-enter the atmosphere "back-end first" so that they feel the braking force (aerodynamic drag) pushing the couches against their backs. In either case, it feels to the astronaut as if the couch is underneath them, and they are lying on top of it. $\endgroup$ Nov 25, 2020 at 15:16
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    $\begingroup$ and I believe that orientation is chosen because the human body is best at surviving large forces in that orientation (when the force is pushing you "up" from your back) $\endgroup$
    – user253751
    Nov 25, 2020 at 16:02
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    $\begingroup$ @Martian2020 Yes. Or rather, if I add some of the content from Solomon's comment, the force is pushing backwards with respect to the velocity of the craft, and by corollary, the craft pushes backwards with respect to the velocity on the people. As Solomon points out, we intentionally orient the people when designing reentry capsules such that this force is in our strongest direction -- pushing flat into our backs. $\endgroup$
    – Cort Ammon
    Nov 25, 2020 at 18:30
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    $\begingroup$ @CortAmmon Very true. Even just taking a breath in such circumstances would be really problematic. I guess the fact that our bodies are asymmetrical front-to-back is advantageous for space flight. $\endgroup$
    – JimmyJames
    Nov 25, 2020 at 21:24
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This is the "braking acceleration" from atmospheric resistance that slows down the descent. The capsule is specifically shaped to provide sufficient but not too great deceleration. See the article Returning from Space: Re-Entry on the web.

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  • $\begingroup$ Thank you for some hint! $\endgroup$ Nov 25, 2020 at 4:09
  • $\begingroup$ I understand it could produce the effect. Would you expand you answer a little bit to get rid of "maybe"? then I can accept the answer. $\endgroup$ Nov 25, 2020 at 4:17
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    $\begingroup$ this should be a comment. Answer only when you are sure. For discussion you should use the comments. $\endgroup$
    – Anonymous
    Nov 25, 2020 at 4:18
  • $\begingroup$ I added next phrase "for quite a lot time" and now I'm not so sure about braking. Is it ON for long? $\endgroup$ Nov 25, 2020 at 4:27
  • $\begingroup$ @Martian2020 Yes, the braking happens over basically the entire descent (until the aerodynamic forces get too low due to low speed, at which point they open the parachutes). If it didn't, you'd have much higher braking forces that would be much more dangerous to the astronauts. $\endgroup$
    – Hearth
    Nov 25, 2020 at 16:26
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From a general relativity point of view, no objects, free falling or otherwise, are subject to a force of gravity, as gravity isn't a force, it's a manifestation of the curvature of space time. But to the extent that there is such a thing as gravitational force, objects in free fall are subject to it just as much as other objects.

Now, the way we generally experience gravity is not directly. Rather, we experience the normal force between us and the ground that results from and resists gravity. A person in free fall won't feel gravity, in that there will be no normal force resisting gravity, and so they won't feel anything pushing on them, but they are accelerating with respect to the Earth's reference frame, and so in that sense are undergoing a force.

If someone undergoing re-entry is subject to a braking force that causes them to accelerate five times as much as Earth's surface gravitational acceleration, then they are subject to a force that is five times the normal force that would be required to resist gravity. Since the normal force is equal in magnitude (though opposite in direction) to gravity, this is equivalent to saying that they are subject to a force five times the gravitational force.

It might be a bit more accurate to say that they are subject to five times gravitational acceleration, rather than force, as the former is an intensive property and the latter is extensive, but the latter is still a reasonable thing to say.

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