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Himanshu Raj
2007-Jun-30, 01:59 AM
We all know that when astronauts go into space they experience Zero Gravity. But the gravity of Earth is always present there. One should experience some weight over there due to the Earth's gravity. Still they experience Zero Gravity. Why does this happen? Is it because of the astronaut's revolution around the earth or does the Earth's gravity becomes weak enough up till that point.

I have seen in few Hollywood films that when astronauts re-enter Earth, objects in the spacecraft start falling in the direction towards the Earth. How is that true if the spacecraft is falling freely. Freely falling objects experience Zero Gravity.

Noclevername
2007-Jun-30, 02:11 AM
The term "zero gravity" is misleading. If an object is in orbit, then technically it is in free fall. A person inside an orbiting craft may experience weightlessness (floating around the cabin), but is not actually outside of Earth's gravity field. Skydivers who reach terminal velocity also experience the sensation of weightlessness.

The technical term for "zero" G is "microgravity", since anywhere in the universe has a little gravity. But Zero-gravity is the popular term.

And when you re-enter the atmosphere, Earth is suddenly dragging you along with it, you are no longer in free fall but just a plain old fall.

Noclevername
2007-Jun-30, 02:35 AM
Here's some info:
http://en.wikipedia.org/wiki/Free_fall
http://en.wikipedia.org/wiki/Weightlessness

Himanshu Raj
2007-Jun-30, 02:46 AM
Suppose that I were in a spaceship on an interstellar (or an intergalactic) journey. Unfortunately I encounter a Black hole in the path of my journey. The black hole will pull on the spaceship and I will be under free fall condition. There would no drag force on the spaceship the space is completely vacuum. The objects in the spaceship will still be floating and will not start to fall in the direction towards the BH. Suppose that my spaceship does not have any instrument that that could infer the presence of the BH. Then there seems to be no way to detect the presence of the BH form the inside of my spaceship. Again weightlessness is the factor responsible.

Noclevername
2007-Jun-30, 02:57 AM
Suppose that I were in a spaceship on an interstellar (or an intergalactic) journey. Unfortunately I encounter a Black hole in the path of my journey. The black hole will pull on the spaceship and I will be under free fall condition. There would no drag force on the spaceship the space is completely vaccume. The objects in the spaceship will still be floating and will not start to fall in the direction towards the BH. Suppose that my spaceship does not have any instrument that that could infer the presence of the BH. Then there seems to be no way to detect the presence of the BH form the inside of my spaceship. Again weightlessness is the factor responsible.

Your ship's sudden change in course would be a pretty good indicator. And there are in fact devices, such as the Forward_Mass_Detector (http://en.wikipedia.org/wiki/Forward_mass_detector#Forward_Mass_Detector), which can detect gravitational sources in microgravity conditions.

Himanshu Raj
2007-Jun-30, 03:05 AM
Your ship's sudden change in course would be a pretty good indicator.

As I already said in my post "I encounter a Black hole in the path of my journey". ie the straight line in which I am moving. There seem to question arising for the sudden change in course of the spacecraft.

Thanks for the link that you gave me.

Noclevername
2007-Jun-30, 03:09 AM
As I already said in my post "I encounter a Black hole in the path of my journey". ie the straight line in which I am moving. There seem to question arising for the sudden change in course of the spacecraft.

If it is a gravity source it will still be noticeable from the motion of the interstellar medium. And it will still show up on a mass detector, even if you are moving straight towards it.

Himanshu Raj
2007-Jun-30, 03:19 AM
If it is a gravity source it will still be noticeable from the motion of the interstellar medium.

How can we notice the motion of the interstellar medium? And what kind of interstellar medium? I can't get around your point :doh:. Do you mean redshifts or other sort of weird things that BH produce as the effect due to gravitational sources?

cjl
2007-Jun-30, 03:51 AM
You could see it visually from the effects on nearby light. In addition, when you got close enough, tidal effects would be measurable, though by the time this happened, it would be far too late to do anything.

Lord Jubjub
2007-Jun-30, 03:58 AM
The reason, though, that astronauts feel gravity return as the space shuttle descends, is that the space shuttle is not descending in free fall. It is using the atmosphere to slow down.

Himanshu Raj
2007-Jun-30, 04:02 AM
Yes! When we re-enter Earth then the free fall is hindered due to the Earth's atmosphere. But in the depths of space there are no drag forces to act upon and so there will be a free fall condition

nauthiz
2007-Jun-30, 05:13 AM
Suppose that my spaceship does not have any instrument that that could infer the presence of the BH. Then there seems to be no way to detect the presence of the BH form the inside of my spaceship.

Assuming there's no way to look outside the spaceship, the only way I can think of to detect the presence of a nearby large mass would be to detect the tidal force (http://en.wikipedia.org/wiki/Tidal_force) from it using something like the Forward Mass Detector that was mentioned in an earlier post. Otherwise you wouldn't notice the gravity because, since you're in free fall, everything in the ship would accelerate toward the BH at the same rate.

Himanshu Raj
2007-Jun-30, 11:46 AM
"Otherwise you wouldn't notice the gravity because, since you're in free fall, everything in the ship would accelerate toward the BH at the same rate."

Yes! that's what I wanted to rectify. I think the hollywood film-makers should make clear cut illustrations of objects starting to fall in the direction towards that massive objects.

Thank you nauthiz for the link.

antoniseb
2007-Jun-30, 01:48 PM
I think the hollywood film-makers should make clear cut illustrations of objects starting to fall in the direction towards that massive objects.

I try not to build my science understanding from what cinematic story-tellers decide to show on the screen. BTW, in the ISS, this differential gravitation does not move objects very rapidly, so showing this movement on film would be like showing rust forming on film (less excitement than watching someone sleep).

Jens
2007-Jul-02, 07:59 AM
Skydivers who reach terminal velocity also experience the sensation of weightlessness.

I'm guessing (I've never done such a silly thing myself as jump out of an airplane) that skydivers would have the sensation of weightlessness as they move toward terminal velocity, but that gravity would return as their speed stabilizes. The first few moments are essentially free fall, but then the weight of the air against your body would become 1G, I think. Just like when an elevator is descending, you don't feel lighter.

trinitree88
2007-Jul-02, 11:12 AM
I'm guessing (I've never done such a silly thing myself as jump out of an airplane) that skydivers would have the sensation of weightlessness as they move toward terminal velocity, but that gravity would return as their speed stabilizes. The first few moments are essentially free fall, but then the weight of the air against your body would become 1G, I think. Just like when an elevator is descending, you don't feel lighter.

Jens. You are correct about the terminal velocity of the sky diver. You want the words.." descending at constant velocity" for the elevator too. When it starts from rest, and accelerates towards...finally reaching it's constant velocity downwards....you do feel lighter.
pete

joema
2007-Jul-02, 11:46 AM
...when astronauts go into space...the gravity of Earth is always present there. One should experience some weight over there due to the Earth's gravity. Still they experience Zero Gravity...does the Earth's gravity becomes weak enough up till that point...
The earth's gravity is only slightly weaker at a 200 mile orbital height than on the surface. If you were sitting stationary on a pole 200 miles high, you'd weigh nearly the same.

You are never outside the earth's gravity, and there is no distance beyond which the earth's gravity doesn't affect you. Even now, your body is being attracted by the Andromeda Galaxy, 2.2 million light-years away.

Statements about spacecraft "escaping the earth's gravity" doesn't mean they've escaped the influence of that gravity, since the earth's gravity affects them no matter how far away they travel. Rather it means they've reached a speed whereby another celestial body (e.g, the moon) will eventually attract them more strongly than the earth.

A "g meter" is a common instrument for measuring gravitational acceleration in an aircraft or spacecraft. It will read essentially zero in orbit, hence the term "zero g", and John Glenn's famous statement "zero g and I feel fine".

It's true in orbit there are minute, microscopic residual gravitational forces such as the gravity of the spacecraft itself, so the term "microgravity" is more recently used. However the same is true for most other areas of physical science or engineering -- in almost no case where a measurement or quantity is described as zero is it really mathematically zero to many decimal places.

Extravoice
2007-Jul-03, 05:05 PM
As I already said in my post "I encounter a Black hole in the path of my journey". ie the straight line in which I am moving. There seem to question arising for the sudden change in course of the spacecraft.

Thanks for the link that you gave me.

Assuming that you have a way of tracking the progress toward your destination, you might notice that you're getting there faster than expected.

Noclevername
2007-Jul-03, 05:59 PM
Technically, you're never at "zero" gravity, as the mass of the spacecraft is always present.

hhEb09'1
2007-Jul-03, 07:01 PM
Technically, you're never at "zero" gravity, as the mass of the spacecraft is always present.and the Andromeda Galaxy :)

Nicolas
2007-Jul-03, 08:26 PM
I'm guessing (I've never done such a silly thing myself as jump out of an airplane) that skydivers would have the sensation of weightlessness as they move toward terminal velocity, but that gravity would return as their speed stabilizes. The first few moments are essentially free fall, but then the weight of the air against your body would become 1G, I think. Just like when an elevator is descending, you don't feel lighter.

That's one of the reasons I don't like the free use of "free fall". When you fall into a stationary atmosphere, you'll never experience zero G as you'll always be decellerated by drag, upto the point where drag equals gravity and you feel 1G air pressure on you.

In an atmosphere that moves along with you, or is absent altogether, you can experience true zero G as no drag forces pushes on you, you and the air surrounding you fall at the local gravity acceleration.

Bearded One
2007-Jul-04, 04:41 AM
Technically, you're never at "zero" gravity, as the mass of the spacecraft is always present.

and the Andromeda Galaxy :)Hmmm, what would happen if you had a ship such as H.G. Well's cavorite powered craft? Cavorite blocked all gravitational forces, the principle was you enclosed the craft in cavorite and then opened panels in the direction you wanted to go. The gravity would pull you in that direction. Problem is that all other sources would be blocked, there would be no countering pull to balance things out. You would feel the gravitational attraction of the whole Universe in that direction, with no balancing pull from the rest of the Universe. How fast would you accellerate? :think:

Half Wit
2007-Jul-04, 04:45 AM
Technically, you're never at "zero" gravity, as the mass of the spacecraft is always present.

It's a little late in the evening for me to be doing calculus problems, but if the spacecraft is spherical, hollow, and uniform, won't there be zero gravity (discounting the influence of any objects external to the spacecraft) at all points on the interior?

Himanshu Raj
2007-Jul-04, 06:04 AM
It's a little late in the evening for me to be doing calculus problems, but if the spacecraft is spherical, hollow, and uniform, won't there be zero gravity (discounting the influence of any objects external to the spacecraft) at all points on the interior?

Yes. Gravitational Feild inside a uniform sperical shell is always zero- the shell theorem.

cjl
2007-Jul-04, 08:09 AM
Hmmm, what would happen if you had a ship such as H.G. Well's cavorite powered craft? Cavorite blocked all gravitational forces, the principle was you enclosed the craft in cavorite and then opened panels in the direction you wanted to go. The gravity would pull you in that direction. Problem is that all other sources would be blocked, there would be no countering pull to balance things out. You would feel the gravitational attraction of the whole Universe in that direction, with no balancing pull from the rest of the Universe. How fast would you accellerate? :think:

Not that fast - the force drops off as distance squared, and most stuff in the universe is VERY far away...

joema
2007-Jul-04, 12:35 PM
Not that fast - the force drops off as distance squared, and most stuff in the universe is VERY far away...
That is true, from a gravitation standpoint alone.

However, as described by H.G. Wells, the initial acceleration would be extremely high due to atmospheric effects.

Imagine a vehicle plated with Carvorite, Wells' hypothetical gravity screen. Carvorite cuts off gravity in a vertical column above it, which causes the air column and vehicle above the Carvorite screen to have zero weight. Yet the air beneath still has weight, and is pressing on the vehicle at atmospheric pressure -- 14.7 psi (101,000 newtons per square meter).

If the vehicle was (say) 5 meters in diameter, the atmospheric pressure on the underside would be exerting about 1/2 million pounds of force (2 million newtons) of upward thrust.

If the vehicle weighs about 50,000 lbs (22,679 kg), disregarding atmospheric friction, the initial acceleration can be approximated from:

f=ma, where
f=force in Newtons (2,000,000 N)
m=mass in kg (22,679 kg)
a=acceleration in m/s^2

a=f/m
a=2,000,000 / 22,679
a=88 meters/sec/sec (8.9 gs acceleration)

Once in space this effect would cease, leaving only the acceleration due to attraction of other bodies.

How rapidly would a Carvorite-screened vehicle accelerate toward the moon from the earth's vicinity?

This is given by the formula a=GM/r^2. There's a simple calculator for this here: http://www.ajdesigner.com/phpgravity/gravity_acceleration_equation.php

Lunar mass is 7.36E22 kg, distance to the moon is about 386,000 km. That gives an acceleration of: 3.4E-5 m/sec/sec (.001 inches/sec/sec), which is very small.

The attraction of more remote celestial bodies would be tiny.

However -- you'd presumably be moving at a very high velocity upon leaving the earth's atmosphere due to the previously-described affect. If headed in approximately the direction of the moon, after a while you'd be close enough for lunar gravity to have an effect, considering the vehicle would be screened from earth's gravity.

The same technique wouldn't work for going to Mars unless assisted by other propulsion methods. The distance is just too great and the gravitational attraction of those bodies too weak.

Bearded One
2007-Jul-04, 02:42 PM
Cavorite was supposed to stop all gravitational forces. I guess I always had a problem with that, not sure how that would affect things. I'm not even sure what that means, would the inside be true zero gee? Whatever that is. If plotting the area graphically would the inside be on top of an infinitely high plateau with vertical sides? Stepping off the edge of an infinitely high plateau does not seem like a healthy thing to do. :lol: