[Lord Soth]

Rather than hijack Black echo's topic i started a new one.

Now fundamentally this may be a very dumb question BUT.....

I do not understand where G's come from in space. I was under the impression that everything is weightless in space. So if no weight how come you get G-force during acceleration etc in space.

It always seems to be a topic that is taken for granted in sci-fi books and RL but i have never fully been able to work past the problem.!!!

Anyone who would be kind enough to explain (in reasonably simple terms http://www.sffworld.com/ubb/smile.gif ) I would much appreciate it.

[blahginger]

In space you are weightless, but not massless. F = MA (Force = Mass * Acceleration).

To take this back one step...you can think of your weight being the accelertaion of your mass against the Earth. Gravity (9.8 meters/second[squared] ) is really an accelleration.

I hope this makes sense, (and I hope that I got it right).

[This message has been edited by blahginger (edited October 13, 2001).]

[Penumbra]

Mass in outer space is always referenced to some point that is within the gravitational matrix of a star, regardless of where it is. Therefore, if you are floating in space, and move, it is referenced to a fixed point. But, if you are in a craft, it undoubtedly has its own simulated gravitational field (gyroscope) in which a G force is referenced to it. If you are approaching an object, your G is compared with it.

[Sammie]

Force = mass x acceleration

can be rewritten as:

Weight = mass x 'acceleration due to gravity'

where acceln due to gravity is equivalent to the 'gravitational force' exerted by a body (ie 9.8 m/s/s on Earth)

These two equations are essentially one and the same.

UNITS:
weight/force = newtons
mass = kg
(acceln due to) gravity = m/s/s (metres per second squared)

Any body has a CONSTANT mass, wherever it is in the universe. Mass refers to the mass of the body in comparison with that of a 1 kg standard. If you take this standard 1kg block to the planet X with you, your mass COMPARED WITH THE STANDARD will be the same as it was on Earth. So: mass of a body is constant.

Any mass exerts a gravitational pull. In the case of you and me this is infinesimally small - completly negligible - in the case a mass as great as that of a star or planet it is somewhat more significant. But it is always there.

The size of this gravitation pull is inversly related to the distance between the mass generating the pull and the body it acts on. The further you go from the Earth, the smaller the strength of its gravitational pull upon you. HOWEVER, this relationship (which i am NOT going to define further) works in such a way that:

NO MATTER HOW SMALL THE MASSES OF ANY TWO BODIES, IN ORDER FOR THE GRAVITATIONAL ATTRACTION (acceln due to gravity) BETWEEN THEM TO EQUAL ZERO, THE BODIES MUST BE AN INFINITE DISTANCE APART.

We can prove that the universe is expanding, therefore the universe cannot possibly be infinite (since an infinite body by definition cannot get any larger).

If the universe is finite, then no two bodies in the universe can ever be an infinite distant apart. THEREFORE: every body in the universe exerts a gravitational pull, however infintesimal, on every other body in the universe. THEREFORE: there is no escaping Acceleration due to Gravity, not anywhere in the universe.

SUMMARY:

1) Anything that has mass on Earth (ie spaceman, spaceship) has mass EVERYWHERE in the universe
2) Anything with mass experiences acceleration due to gravity, no matter what its location in the universe.

Weight = mass x acceln due to gravity

1) and 2) esstablish that, for a body with mass on Earth, neither mass nor acceln due to gravity can ever be zero, so/

3) A space man/spaceship etc (or any other body)ALWAYS HAS WEIGHT, anywhere in the universe.

Right. So, no-one in space is ever weightless. (If THAT's not sufficient proof then borrow a physics textbk, but be warned, it'll be even less comprehensible than me!)

The concept of 'weightlessness' in space is a misconceived one, caused by observances of astronauts in 'free fall' inside orbiting space craft. This is in fact a far more accurate term. The appearance of weightlessness/lack of gravity is caused because both spacecraft and contents (inc. astronauts) are constantly falling back towards the Earth WITH THE SAME VELOCITY (velocity being a vector quantity refers both to speed and direction of motion). There are no forces acting on the astronauts, WHEN COMPARED ONLY WITH THE SPACECRAFT AND OTHER OBJECTS FALLING WITH THEM. The spacecraft doesn't get any closer to the Earth because of the tangential force (ie of the propulsion system) acting, simultaneously, at right angles to the gravitational pull. The sum of these vectors is an overall force moving them in a circle around the earth at a constant distance - as long as the mathematicians did their job properly when they worked out the timing of those fuel blasts anyway......

Okay....weight always present for mass within finite universe....weightlessness = myth caused by misunderstanding of free fall.....that it!

Long pause as Sammie realises that, whether or not she has actually managed to IMPART any of her knowlege, SHE understands all this and perhaps she should be studying astrophysics - not medicine, which is, sadly, frequently beyond her.

[karljah]

Doesn't matter about weightlessness. What G is is not only a measure of the acceleration of the ship (9.81 m/s squared= 1G), but the inertial affect YOU feel (in a ship going 1G, you hit the back wall at 9.81 m/s squared). You go 1G, you feel like you are in Earth normal gravity. 2G, twice Earth normal, and youship is moveing at 19.62 m/s squared, It's a measure of your ships acceleration, and how YOU feel about it. Build the right ship, youcan accelerate as fast as you want, but humans can't stand high Gs.Go 10 Gs, and the ship is MOVING, but the Humans inside have passed out, and might be dead. Does this make sense? I can explain it better when I'm sober...

[Sammie]

but the humans inside the spaceship are in the same frame of inertia as the spaceship. They are not accelerating RELATIVE TO THEIR FRAME OF INERTIA - therefore they shouldn't feel any force acting on them. For example, the air within the space ship is moving at the same velocity as the people, so they won't feel any force of air pushing against them - they are not moving THROUGH the air surrounding them, only with it.

I think.

Does anyone on this board have more than A-level standard physics? like a nice useful degree?

Sammie

[This message has been edited by Sam82 (edited October 14, 2001).]

[blahginger]

Sam82, When in a ship you will only feel the force when accelerating. Just like an airplane, you feel the force when taking off, but once you reach a constant speed you no longer feel any force. Technically your frames of reference when accelerting are not the same, your frame is being pushed along by the ships frame. The air in the ship is also being pushed along, but since the air has a much lower mass than you it "feels" less force.

[Lord Soth]

Well that debate has now moved well out of my understnading http://www.sffworld.com/ubb/smile.gif But many thanks for the imparting of knowledge! It has answered a long standing question. Thank you all http://www.sffworld.com/ubb/smile.gif

[Rocketsheep]

Can I try?

The problem explaining gravity's force comes from weight and acceleration being so tightly linked to gravity that the words are used almost interchangeably.

Weight is a force, and a force cannot be felt if it has nothing to push against.

If you took your bathroom scales to a remote spot between the stars and glued them to your shoes they will read zero. If you attached a rocket to the underside of the scales and started it firing you would feel a sensation of weight. If the thrust of the rocket increased your speed by thirty two feet per second, every second, you would have exactly the same weight you had on Earth.

We measure such forces in terms of so many 'gees', meaning that a person experiencing 10g would feel ten times his normal weight but the actual gravity of Earth is not involved when the force of weight is produced wholly by accelleration, and it is unfortunate that the same word is used to describe an effect that has two different causes.

Earth's gravity could be measured galaxies away even tho its effects would be negligible. So to say we are beyond Earth's gravity is also a misleading statement.

Everyone else just said the same thing but I'm a high school drop out so I need little words and a nice set of scales to get my widdle sheepy brain around it.

[Shehzad]

My aching head!!