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I understand how one can feel weightless in an elevator - in the person's reference frame, they are not being pushed up by the floor.

However, I don't understand how this can occur in parabolic flight. A textbook I found said that "passengers will be objects in free fall and will feel weightless because they are not in contact with the floor of the airplane."

How can this occur? Wouldn't a passenger be fixed to the floor of the airplane the entire time? Simple explanations are very helpful; thanks in advance.

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  • $\begingroup$ Why do you think a passenger would be "fixed to the floor" if the plane is in free fall? $\endgroup$
    – d_b
    Commented Sep 17, 2022 at 23:10
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    $\begingroup$ What is different between an airplane in free fall and an elevator in free fall, other than the airplane has some horizontal velocity? $\endgroup$
    – Not_Einstein
    Commented Sep 17, 2022 at 23:32
  • $\begingroup$ Right, so then why don't we use elevators? $\endgroup$
    – Jean Pierre
    Commented Sep 18, 2022 at 0:13
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    $\begingroup$ Parabolic flights provide approximately 20s of zero-g condition. With a free falling elevator, you would need approximately 2.4km of free fall, without considering the space needed to decelerate the elevator. If you exploits the acceleration even in the ascending part of the trajectory, you could reduce the space needed of a factor 1/4, i.e. 600 m, excluding the space required for accelerating and decelerating the elevator from and to rest to avoid fatal crashes. Quite a demanding infrastructure, for the height (although taller elevators exist, in mines) and for the mechanical components $\endgroup$
    – basics
    Commented Sep 18, 2022 at 1:01
  • $\begingroup$ Side remark: Galileo's Leaning Tower of Pisa Experiment. $\endgroup$
    – Kurt G.
    Commented Sep 18, 2022 at 8:26

2 Answers 2

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The airplane has a relatively constant speed in the horizontal direction but is in freefall in the vertical direction.

Imagine your elevator in freefall example: people inside feel no gravitational force as you stated. Now imagine that the same elevator is given a momentary push perpendicular to its direction of motion, so that it now gains a horizontal component of velocity. It still is in freefall in the vertical direction, but now has a parabolic path.

As far as the vertical direction is concerned, both cases are identical.

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  • $\begingroup$ I'm impressed that some pilots are experienced enough to take their airplane through a parabolic path in the air. $\endgroup$
    – David White
    Commented Sep 18, 2022 at 0:38
  • $\begingroup$ @DavidWhite Yep. Certainly something that would take practice. $\endgroup$
    – joseph h
    Commented Sep 18, 2022 at 0:50
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Imagine you throw a ball upward into the sky. The instant it leaves your hand, it is coasting ballistically along a parabolic path through the air. An ant clinging to the ball will not experience its own weight as the ball follows that path, as the ball and the ant are both in free-fall the whole time.

The pilot in the plane dives to pick up speed and then pulls up to place the plane into an upward parabolic path- the same free-fall trajectory followed by the thrown ball. At that point, the plane and its contents are all following exactly the same free-fall arc and their weight disappears.

The passengers will remain in their seats as long as their seat belts are still fastened, but once they unbuckle them, they will be free to float out of contact with their seats because both their seats and their bodies are following the parabolic free-fall path, as well as the floor to which the seats are bolted.

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