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Assume an object is falling toward the earth then at what velocity would the object be weightless? Taking the air resistance into account might be a factor here so lets assume no air resistance to simplify matters.

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    $\begingroup$ Weightlessness occurs when you are in freefall - that is, when your acceleration is equal to the acceleration due to gravity. Speed is irrelevant - a base jumper becomes weightless the instant they jump, when they're moving at 0m/s. If they were falling in a vacuum, they'd accelerate until they hit the ground, and would experience weightlessness the entire time regardless of their speed. I don't really see any connection to escape velocity or how this is a "reverse". $\endgroup$ Commented Jan 10 at 18:10
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    $\begingroup$ Iv'e voted to close, you clearly have a big misconception about what "weightless" means. I think Nuclear Hoagie has started to unpack this, but I don't think an answer is possible to the question as written. $\endgroup$
    – James K
    Commented Jan 10 at 18:53
  • $\begingroup$ I edited my question to make it more clear and remove misconceptions and other problems. Thank you. $\endgroup$
    – Sedumjoy
    Commented Jan 10 at 22:33
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    $\begingroup$ Even when the question is by its wording based on false assumptions, you should not delete it. This misconception is reasonably common. This having it here and an answer is helpful. $\endgroup$ Commented Jan 10 at 23:44
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    $\begingroup$ A note on comment reply. Please don't put space between @ and the name. Otherwise, that person won't be notified. $\endgroup$ Commented Jan 11 at 4:51

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There's no speed at which you experience weightlessness; to stay weightless, you need to keep getting faster and faster all the time.

From our perspective on Earth, everything accelerates downward at about 9.8$\frac{m}{s^2}$ ($32\frac{ft}{s^2}$). Weightlessness is what happens when you're accelerating at exactly that rate, whether it's because you jumped off a rock or you're in orbit (which is just falling while moving forward fast enough to keep missing the planet). As long as you're accelerating downward as fast as gravity will let you, you're in freefall and experience weightlessness.

That's pretty difficult, though. A skydiver starts out in freefall, and they feel weightless for a few seconds, but the wind resistance (or "drag") on their body quickly grows, reducing their acceleration and increasing their apparent weight until they're eventually not accelerating at all. We call that "terminal velocity", and it's usually around 53 meters per second (120 miles per hour) in the spread-eagle position. At that point they're not weightless anymore; their body is experiencing the same weight as if they were laying face-down on a bed, just with a lot more wind.

There are ways to make it work, though. The famous "Vomit Comet" aircraft are designed to fly in an arc that lets them maintain that perfect acceleration for a short period despite the drag trying to slow them down, and inside the cabin, the passengers can float around as if they're weightless. (The name comes from the fact that weightlessness often makes people feel sick at first, as their body isn't used to that feeling lasting for an extended time.)

If there were no air and you were to bring yourself to a perfect stop many miles above the surface, you'd be weightless all the way down, getting faster and faster without limit until you hit the ground at some incredible speed and left quite a crater behind.

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    $\begingroup$ Thank you Darth. This clears up my misconceptions. The light bulb went off as soon as I read your second paragraph. I appreciate you all. $\endgroup$
    – Sedumjoy
    Commented Jan 10 at 22:32
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    $\begingroup$ The'vomit comet'offers about 20s of zero gravity per parabola. Been there, done that. For minutes you need either a supersonic jet or a sub-orbital sounding rocket which reaches heights of about 300km. (100km usually is called the border to space, the iss is at 400km) $\endgroup$ Commented Jan 10 at 23:46
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    $\begingroup$ See also airzerog.com/scientific-research-services $\endgroup$ Commented Jan 10 at 23:53
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    $\begingroup$ If you fell to an airless Earth from a height of 100 km, you'd land with a speed of ~1400 m/s ~= 5000 km/h $\endgroup$
    – PM 2Ring
    Commented Jan 11 at 3:54
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    $\begingroup$ One can "stretch" it by not flying 0g but 'cheat' a little bit without jeopardizing the result of such medical experiments overly, e.g. by flying 0.05g or 0.1g or similar. The quality of µg is anyway "only" around 0.01g rms IIRC, and 0.001g on good days / flights. The duration of each parabola in the flight with lunar (0.16g) and marsian (0.35g) parabolas was considerably longer. But evidently 20...25 seconds already suffized for people to practise stitching wounds. $\endgroup$ Commented Jan 11 at 17:51
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Any speed. If you free fall then you feel weightless. That begins at zero speed, when you start falling, and ends when you hit something at arbitrarily high speed.

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  • $\begingroup$ If you're going fast enough sideways, you can keep missing the planet despite gravity accelerating you towards it, avoiding the sudden stop at the end. (This is a fun way to describe orbiting. How fast? what-if.xkcd.com/58 covers low orbit.) $\endgroup$ Commented Jan 12 at 13:40
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In general, you are weightless unless an electromagnetic force acts on you. (What we call "contact" with other matter like air or the ground is electromagnetic interaction of electron hulls. That is the reason you had to assume a vacuum in your question. An electrostatically or magnetically levitated body on Earth would also feel weight.)

That is more obvious and expected when you are far away from planets and stars, "floating in space"; even though there is still lots of gravity around from galaxies and stars, we would not feel it and would not perceive any motion with our bare senses due to the large distance of any reference points.

But it is no less true in close proximity to Earth, for example on the ISS (tidal forces due to an inhomogeneous gravitational field which pulls on some parts more than others aside). In both cases — far away or close by — you follow a geodesic, the equivalent of a straight line in curved spacetime. The line is "straight" because no forces act on you (and the gravitation is modeled as spacetime curvature).

In a way, this inverts the notion of "acceleration" because one could argue that one is "unperturbed" when following a geodesic; by contrast, when we stand on Earth, the ground constantly pushes us "out of our natural way", accelerates us, if you want. That is the meaning of Einstein's elevator Gedankenexperiment. Inside a black box like the elevator you feel (and are, in the relativistic sense) accelerated when you stand still but feel (and are, in the relativistic sense) standing still when you free-fall in what appears an accelerated motion to an outside observer.

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Weightlessness isn't something you experience when you're falling by yourself. The fact that you're falling means that gravity is pulling you down, and that's due to your weight. Instead, you feel weightless when you're falling at the same rate as whatever container you're in.

If you were in an airplane and its wings fell off, it would start falling. But air resistance would limit it to some terminal velocity, so it won't fall as fast as gravity is trying to accelerate the plane and its passengers. So you would still experience some upward force from the floor and seat, and wouldn't be weightless.

You may feel almost weightless when you ride a roller coaster. The designer should minimize the friction of the wheels, so you accelerate at near 1G during the big drop.

To experience weightlessness, the Vomit Comet uses a powered nosedive to accelerate down at the same rate as gravity. And a roller coaster could make you feel this by powering the car at high speed rather than just allowing it to drop down the track; it might even go faster than gravity would have accelerated it, and you'll experience negative weight -- you'll feel a push against the belt or bar that's holding you in your seat.

In these cases you notice the weightlessness because there's nothing keeping you pressed against the floor of the aircraft or car. Your fall due to gravity is the same as the descent of the container, so you can float freely relative to it.

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    $\begingroup$ (slightly more than) half of the 0g time in a parabolic flight is on the ascending path of the parabola. Going down is by no means a prerequisite for weigthlessness (yet another very common misconception). $\endgroup$ Commented Jan 12 at 21:08
  • $\begingroup$ Good point. When the plane's upward velocity slows down at the same rate as gravity, the passenger's upward velocity reduces at the same rate due to gravity. $\endgroup$
    – Barmar
    Commented Jan 12 at 21:14
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~8km/s as close to Earth as practical, less when higher.

This is how orbital motion works. It is essentially a freefall, so in orbit you feel weightless.

But the essence is not the speed. It is the acceleration. See the other answers.

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  • $\begingroup$ When the trajectory is straight you'll feel just normal gravity. Speed is not only not the essence: It is irrelevant. $\endgroup$ Commented Jan 12 at 8:44
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There isn't any velocity where the object would be "weightless".

A simple example is riding in a lift/elevator, at the start and end of the lift's journey you experience a moment of increased and reduced gravity while accelerating and decelerating compared to the direction of gravity but during the middle when the lift is at a constant velocity you experience the same 1g as someone on the ground.

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