why does astronaut feel ' no weight ' when in space?
A person in free-fall has all the bits of his body falling at the same rate. For this reason the astronaut does not sense that parts of his body want to move relative to other parts. But a person can be weightless in the Earth's atmosphere in free-fall too. This is how the train the astronauts. They have a special jet, which is often referred to as the vomit comet which is used soley for this purpose. The plane reaches a particular altitude and then dives at a rate of 9.8m/s2. The occupants are also falling at that rate to so if they are in the center of the jet at one time (off the floor etc) they never move from there unless there is a bit of drift. In fact this is how they filmed that movie Apollo 13.
If you're in an elevator car (lift) and the supporting wires break (and any safety backup systems fail), you will be in free fall (till the car hits the bottom of the shaft). This would work best if there was no air in the shaft and the car didn't touch the sides - that way all friction would be eliminated.
Depending on the height of the building, you would enjoy quite a few seconds of weightlessness, just the same as an astronaut does. Unfortunately you would have to pay dearly for the experience after less than a minute :tongue:
Is there NO way of determining whether you are actually in a gravity free enviroment or just experiencing a free fall (in both cases you fell no weight)?
Free fall is locally indistinguishable from a gravity free environment within some specified accuracy. However, free fall can be identified using the fact that the gravitational ``field lines'' are not perfectly parallel. Hence, free test particles will depart from inertial paths by detectable amounts over significant space-time displacements.
you feel no "weight" in space because you can only find the weight of an object when gravity is present. Weight is not a constant figure...the higher you climb in altitude the less you will "weigh". So you feel no Weight" in space because weight does not exist without the forces of gravity acting upon the object in question. Your mass however will stay constant regardless.
Also (in the elevator car scenario) your feet are slightly closer to the Earth than your head, and so experience a fraction more gravitational force. The net result is a very slight force tending to stretch you lengthwise. These tidal forces would probably be too small to measure though, given the scale of the human body and the Earth.
Such tidal stretching forces do become important with immensely strong gravitational fields, as you might find in the vicinity of a neutron star or black hole.
What happens if you lift an object from the bottom to the top of this free-falling elevator; you are doing work against gravity? Is this non-sensable because is the whole elevator is accelerating at rate g?
No, just against its inertia - but when the elevator gets to the bottom of the shaft, you will need to do some work to stop it...
Btw, I visited Aarhus with the US Navy a few years ago. Beautiful town.
As others have explained throughout this thread, the experience of "weightlessness" has nothing to do with the absence of gravity. You can experience weightlessness anytime you like right here on earth--albeit briefly--by just jumping into the air.
Astronauts don't feel gravity while in orbit, because it is being used to keep them in orbit, and at their certain speed and altitude is not strong enough to actually do more than that (or less).
This is not to say that gravity does not work on them. On the contrary, gravity oppose their straight line movement, it is changing their direction all the time, otherwise they would have been going at a straight line into space.
If you'll drive fast enough on the street with your car, you will become weightless too, just like the astronauts. (friction and car speed limit will realistically deny this, off course)
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