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Ignoring that you can't survive in a vacuum.

If you were to jump of a ledge in a vacuum (in a vertical tube eg). While falling would you feel weightless? Surely the earths gravity is still pulling down on you and you therefore feel your weight?

Now if you travelling upwards on a platform which suddenly stops, and it was at such a speed that you carry on travelling upwards yourself (still in a vacuum) would you feel weightless.

(the thing that I don't understand is why you feel less than your weight when standing in a fast moving lift that is deccelerating, knowing the two extremes will help)

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Break your own question to find the answer. 1. What is the meaning of your weight? 2. Why do not you fall (sink) into a firm floor? 3. But you cannot stand on muddy land. 4. You cannot walk on the water. 5. You cannot float on air. 6. You fall in your imagined tube. Your weight is the force that the resisting body exert on your body, most of the time from under your (sole of) feet. Or reciprocally your body exert to that foreign body. Harsh solid floor stands against you. So you feel weight. Mud exert less so you dip into it until you get to an equivalent force. Water exert less so you feel light and can lift other people easily. On the air you need a very big parachut to gather tiny forces of air to help reduce your accelerating falling and reach to a safe speed. In the imaginary tube there is no other "body" to exert a force to your body and you do not feel anything against your weight. You are weightless! On a scale, a spring stands against you but you can push it, push it, until your forces become equal. Then the spring tells you what is your weight? Now if you create a force that works in favour of you or against you, then you feel less weight or more weight accordingly. Lift and you are not joint together and if lift starts to change its forces you might get different feelings. Your weight is your mass (the matter that you have with wherever you go) times the acceleration that your body experience at any moment. This acceleration most of the time is a constant value that depends on the mass of the earth (almost 10 meters/seconds/seconds). In a theme park they fasten you firmly to the apparatus. In the lift you are not riveted to the lift so they make the changes of acceleration mildly such that you do not feel bad. When the lift accelerates upward it adds to the forces exerted to your body so you feel heavier. When the lift accelerates downward it seems (drastically said) you are on water. You cannot follow it due to having your own inertia so looks it becomes empty under your feet. If it accelerates downward with (10 meter/seconds/seconds) like a theme park or aeronauts training airplanes that you feel completely weightless. You can find similar interesting questions from "University Physics" by Resnick-Halliday.

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If you are falling you feel weightless even though you are still being affected by gravity - the sensation of weightlessness is just that there is nothing around you to press against you.

An astronaut in orbit is effected by gravity almost as much as you are (being 10% further from the centre of the earth doesn't make gravity much less)

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Emm, but gravity is not a real-force, right ? "Force of gravity" is just a direct consequence of space-time curvature because of the existing mass? So isn't this (!) the real cause you feel weightless (in a vacuum)? – sabiland Oct 4 '11 at 10:12
@sabiland - No it's just a different way of picturing it. Electromagnetism isn't a real force - it's just the exchange of photons. – Martin Beckett Oct 4 '11 at 12:39

I'm sure this is a repeat question, but anyway, a body in free fall doesn't "feel" falling in a particular direction, you feel weightless.

To put it another way - the sensation of having weight in the first place is simply a result of the pressure from the surface that stops your free fall. Let me explain this somewhat biologically:

Every particle in your body is pulled basically identically in the same direction with the same acceleration in free fall (in your "vacuum"), and the surface of the earth or the floor in your elevator is really a bunch of electromagnetic interactions that get in the way and interact with the skin of your feet, which in turn has to interact with your legs, spreading the pressure upwards in your body until it tries to stop the linear movement of the otholiths in your inner ear at which point they bend and provide your brain with the signal that you are not in free fall - which is the same as you feeling gravity or acceleration!

To repeat: any gravity felt is always the result of expending some other force like electromagnetism to "get in the way" of a body's free fall. This is obviously the easy answer to why gravity and an accelerating elevator (as in the classic example) is the same, in both cases the elevator floor is pushing on you.

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Good answers.

My son once asked me if you have to go into space to feel weightless.

I said, no, I can show you it right here and now.

I had my car keys in my hand, and I said watch my keys and my hand.

Then I gently tossed the keys in the air, so that they just stayed slightly above my hand while the keys and my hand went up and then came back down.

He could see that they were floating, weightless, just above my hand.

The "vomit comet" works the same way.

All you need to feel weightless is to not have anything physical supporting you.

Even if you're swimming underwater you don't feel weightless. It may not look like anything's supporting you, but the water is supporting you, by a difference in pressure between your underside and your topside.

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Especially that conection to "vacuum" is silly! – Georg Oct 3 '11 at 17:42

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