gravity... just don't get it!!!

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12 May '07 21:02

eatmybishop
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12 May '07 21:02
i'm sure this is really simple, but i just don't get it... can someone explain it to me...

now, i know that without gravity everything is weightlessness, but does that actually mean the object has no weight whatsoever? after all, it still has mass and density;

without gravity, does every object weigh the same, i cant see how it can, after all, the weight of an object is internal, not external?

in other words, if there was a bus and a feather on a planet with no gravity at all, would the bus be just as easy to push as the feather?
Nicky4815
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12 May '07 21:20
Wieght is defined as Mass*Acceleration due to gravity so in a completely non gravity environment then everything has no weight.
Jirakon
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12 May '07 21:28
Weight is a force. Just like all forces, it needs a mass and an acceleration. Specifically, weight requires a mass and an accelration due to gravity. So yes, everything would be weightless.

in other words, if there was a bus and a feather on a planet with no gravity at all, would the bus be just as easy to push as the feather?

That has nothing to do with weight. That is a question of inertia, which is related to mass. The bus has more mass, and therefore more inertia, so the bus would still be harder to push.
Allderdice
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13 May '07 03:191 edit
It has nothing to do with inertia. The only thing slowing it down will be the air resistance and frictional force. P. S. W=mg, g=9.8m*s^-2, m=mass of an object, W=resulting weight.

P. S. That's for air. bodies in different mediums with the same mass have different weights because of the drag force pushing up (pretty much the same as air resistance)
XZantoth
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13 May '07 07:29
Originally posted by Allderdice
[b]It has nothing to do with inertia. The only thing slowing it down will be the air resistance and frictional force. P. S. W=mg, g=9.8m*s^-2, m=mass of an object, W=resulting weight.

P. S. That's for air. bodies in different mediums with the same mass have different weights because of the drag force pushing up (pretty much the same as air resistance)
[ ...[text shortened]... rong with this statement, but I'm not sure where to start, so just ignore all that was said here.
XZantoth
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13 May '07 07:30
I want to say what's wrong with this statement, but I'm not sure where to start, so just ignore all that was said above.

Sorry for the double post... got my html messed up
FabianFnas
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13 May '07 08:26
I agree with XZantoth. Where should one begin.

The theory of gravitation has been known from the 17th century, thanks to Isaac Newton. It is the part of school basic physics.

It's actually not very hard to grasp but one has to use ones brain a little.
eatmybishop
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13 May '07 08:50
Originally posted by FabianFnas
I agree with XZantoth. Where should one begin.

The theory of gravitation has been known from the 17th century, thanks to Isaac Newton. It is the part of school basic physics.

It's actually not very hard to grasp but one has to use ones brain a little.
so why bother to reply, your post has made no contribution to the thread at all?

also, if you actually read the post by XZantoth and "use ones brain a little" you will find he was referring - not to the original post - but the reply made by Allderdice, or maybe you think html is a science concept 😀!!!
Allderdice
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13 May '07 13:161 edit
Well, if HTML is a science concept - then I have to reconcider everything in this world that I concidered an axiom🙂

Ok, here's the thing:

F=(M1*M2*G)/R^2

where F is gravitational force, M1 and M2 - masses of the objects attracted, G - Newton's Gravitational constant (6.67*10^-11), and R is the distance between the two objects. So, everthing in the universe is attracted to one another, but the value of G is very small, so, you don't feel the attraction between two things of small mass. Mass of the Earth is much greater than the velue of G (5.98*10^24 kg), so you feel a pretty strong gravitational force acting on you.

Also there is another reason for the force to be that small - large distance: technically at this very moment everybody on the Earth is attracted to Jupiter. Despite of its huge mass, the distance squared is enormous, so, you don't feel the force.

P. S. Everything is relative. (c) Albert E.
adam warlock
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14 May '07 17:001 edit
Originally posted by Allderdice
Well, if HTML is a science concept - then I have to reconcider everything in this world that I concidered an axiom🙂

Ok, here's the thing:

F=(M1*M2*G)/R^2

where F is gravitational force, M1 and M2 - masses of the objects attracted, G - Newton's Gravitational constant (6.67*10^-11), and R is the distance between the two objects. So, everthing in t d is enormous, so, you don't feel the force.

P. S. Everything is relative. (c) Albert E.
Albert E. never said that. I still don't know why people repeat that sentence, it's nonsense, and then assume that Einstein said it.
adam warlock
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14 May '07 17:08
Originally posted by eatmybishop
i'm sure this is really simple, but i just don't get it... can someone explain it to me...

now, i know that without gravity everything is weightlessness, but does that actually mean the object has no weight whatsoever? after all, it still has mass and density;

without gravity, does every object weigh the same, i cant see how it can, after all, the ...[text shortened]... ther on a planet with no gravity at all, would the bus be just as easy to push as the feather?
Gravity is an interaction between particles that have mass. So it's not an intrinsic characteristic of a body but something that comes up when that body interacts with another. For instance let's take eletricity: If you could have only one electron in the whole universe and everthing else was to be electric neutral then the electric forces wouldn't be felt. So electric charge is an inerent characteristic of something and electric force to be felt must be to some sort of interaction. The analogy is electric charge->mass (inertia), Coulombs force-> Gravitational force.

in other words, if there was a bus and a feather on a planet with no gravity at all, would the bus be just as easy to push as the feather?
That could'nt happen. The only to a planet not have any gravity at all woulda been to it to be massless. And so far we have no massless planets.

You can think in Newtonian terms, or in the general relativity terms, but everytime you have two bodies that have mass they must interact gravitationally.

I don't know if I was clear enough but you want me to explain something better just let me know.
Allderdice
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14 May '07 19:19
I never said that it was a direct quotation from the words of Einstein, I just said that everything is relative and put a famous scientist behind this phrase to enforce my argument taking into the concideration the fact that Einstein ACTUALLY said it. Please don't post misleading statements if you do not have proof behind them, especially when a person is interested in a subject. This is not a fight for the best explanation and not a good place for destructive criticism.
sonhouse
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14 May '07 21:05
Originally posted by eatmybishop
i'm sure this is really simple, but i just don't get it... can someone explain it to me...

now, i know that without gravity everything is weightlessness, but does that actually mean the object has no weight whatsoever? after all, it still has mass and density;

without gravity, does every object weigh the same, i cant see how it can, after all, the ...[text shortened]... ther on a planet with no gravity at all, would the bus be just as easy to push as the feather?
You are confusing weight with inertia. If you have a rocket way out in space, far from a sun, the mass still requires a rocket to move it.
From the equivalence postulate of Einstein, which says you can't tell the differance between acceleration and gravity, that is to say, if you are in a space ship on earth about to take off and you are inside in a room with no windows, there is no way for you to tell if you are on earth or being accelerated in space under one G of acceleration.
So if you are in space and you have a scale and on earth you weigh in at 100 Kg, and in the spacecraft you also seem to weigh 100 Kg, by definition you are accelerating at one G. So in that framework, without having a window or insrumentation like a camera to show the environment around the ship, you cannot prove to yourself one way or the other the answer to this question: Am I in space undergoing 1G or am I on earth. The mass of the craft and you are the same in either event but you cannot decide which environment you are in (Say if you were unconscious and suddenly found yourself in this room that happens, unbeknown to you, to be inside a spacecraft under 1 G of accel, you may think you were still back on earth, no instrument inside the room can give you a clue)
DeepThought
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14 May '07 22:57
Originally posted by sonhouse
You are confusing weight with inertia. If you have a rocket way out in space, far from a sun, the mass still requires a rocket to move it.
From the equivalence postulate of Einstein, which says you can't tell the differance between acceleration and gravity, that is to say, if you are in a space ship on earth about to take off and you are inside in a room w ...[text shortened]... you may think you were still back on earth, no instrument inside the room can give you a clue)
Ah, but if you are in a spaceship that is accelerating you can tell the difference between that and gravity from a planet. If you take two pendulums with some seperation then the pendulum strings will be exactly paralell to each other on the spaceship, but on the earth they will be at a (very) slight angle due to the fact that the gravitational field lines apparently radiate from the centre of the earth. This doesn't affect the equivalence principle since it applies only to arbitarily small volumes.

An interesting point is that a body in free fall may appear to be accelerating, but this is it's natural state of movement - it travels along a geodesic in space-time, which from our ground based point of view appears as an acceleration. We on the other hand are prevented from moving along a geodesic in space-time by the ground, it requires a force to stop you moving along a geodesic so we are actually accelerating. Which is the other way round from the Newtonian point of view where on the ground there are two balanced forces for an object on the ground gravity downwards and reaction upwards, and gravity generating an acceleration for a falling object.
sonhouse
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15 May '07 06:52
Originally posted by DeepThought
Ah, but if you are in a spaceship that is accelerating you can tell the difference between that and gravity from a planet. If you take two pendulums with some seperation then the pendulum strings will be exactly paralell to each other on the spaceship, but on the earth they will be at a (very) slight angle due to the fact that the gravitational field l ...[text shortened]... y downwards and reaction upwards, and gravity generating an acceleration for a falling object.
I actually knew that but I don't think you could get a real measurement inside the confines of a spacecraft room, say 2X2X3 meters. Gravitometers can be extremely sensitive however, may be able to show a differance if you are on earth, due to mascons under your building, a few meters apart.