# Thread: Does gravity push or pull ?

1. Originally Posted by Sean Clayden
The pressure on your feet is all the weight above it.............
Exactly. Which means something is pulling the bulk of the body's mass downwards, correct? If one were to argue that it's the atmosphere, i.e. air pressure, that pushes an object down, then why does that air "push"?
If I weigh 14 stones, my feet would feel a large proportion of that according to gravity as we know it.

Stand on scales on the moon, what would your weight be ?

About 25% of what we know on earth.
It would be around 16%, actually.
Does that mean the moons attraction is less or the exertion of gravity is less ?

If you were in an accerlerating vehicle, you would feel the exertion of acceleration or increased gravity on your whole body, not just your feet..........
It depends on the body's orientation to the direction of acceleration. Lay face down, and you feel pressure on your face, torso, and the front of your legs. Now compare this to the pressure you feel on the soles of your feet when you are standing up. The latter pressure feels greater, correct? There is more mass in line with the direction of the force... more to work with, so to speak. For gravity to be a pressure, as you suggest, then it would depend more on surface area.
Also, if gravity were an exertion, it would have been impossible for individual stars or planetary bodies to form. All matter would just aggregate.
Last edited by SMEaton; 2007-Jan-24 at 07:54 PM.

2. Originally Posted by Sean Clayden
The pressure on your feet is all the weight above it.............

If I weigh 14 stones, my feet would feel a large proportion of that according to gravity as we know it.

Stand on scales on the moon, what would your weight be ?

About 25% of what we know on earth.

Does that mean the moons attraction is less or the exertion of gravity is less ?

If you were in an accerlerating vehicle, you would feel the exertion of acceleration or increased gravity on your whole body, not just your feet..........
You feel the effect of gravity on the rest of your body as well. Lift your arm up, it has weight--if you doubt it, do it 2000 times and see if you get tired

Sometimes my head seems heavier too, like now

3. Originally Posted by Sean Clayden
From what I understand the moon wasn't always here. It was passing and caught up in within our gravitation and is moving further away from us as we speak.

Global warming or the moon moving away ?
The origins of the moon are still under debate, but the "capture theory" is generally not accepted as valid. It's too hard to account for an object as massive as the moon, travelling at just the right velocity (which would be very slow), being captured by the Earth. The leading "ejected ring" theory, wherein the nearly formed Earth was hit by a Mars-sized protoplanet, creating a ring of material composed of Earth and protoplanet debris eventually forming the moon, still remains under debate and research. We simply don't know.
As for the 'global warming' comment, I'm not sure what you're asking.

4. Originally Posted by hhEb09'1
You feel the effect of gravity on the rest of your body as well. Lift your arm up, it has weight--if you doubt it, do it 2000 times and see if you get tired

Sometimes my head seems heavier too, like now

Good point.

5. I thank you for your time in answering may questions and you have enlightened me on a few points.

You're welcome, glad to have been of help.

It's the dips in space that causes me concern.

I think this is really more a case of getting your head around a tricky concept. We're used to 3D space because we live in it, the idea of being able to bend that space in a 4th dimension often causes trouble because we simply can't visualise it well. This is one of the reasons that using a visualisation of a 2D sheet with bowling balls on it sometimes sort of helps because we can see how the balls "dent" the sheet they sit on.

What I mean is, how is light bent, deflected slowed down by all these forces.

It isn't. Light always travels in a straight line, just sometimes that straight line travels through curved space. Again this can be a tricky concept, so reducing it in dimensions again can help. Get a piece of paper and draw a straight line on it, now bend the paper so it is curved. As far as the line is conserned, it is still straight, but obviously it isn't. The same is true with light. As far as the photon is concerned it is travelling at a constant speed in a straight line in a single direction, that space is bent by large masses causes the light to appear to bend, just as your line on the paper bends.

Light travels at c within a vacum, however our universe seems to have a force all of it's own, how do we know that what we see is correct to our calculations of c as we know it ?

I'm not quite sure what you're after here.

If i could create a room within a vacum, of infinite size and threw a number of obects into it. Would these objects according to there size, mass, volume create/act as what we now as a univese/Galaxy over an infinate period of time.

Well size and volume I expect are the same thing, but otherwise, would they act in accordance to what we see in the universe? I'd expaect them too, yes. I don't know that they would create a galaxy/universe as we see it, but that really depends on what they were made up from. If it was a huge amount of hydrogen, then if is quite likely that over time (a few billion years) you'd start to see many similar structures (stars/galaxies) to what we do, if they were large lead balls, then you are likely to get something totally different happening simply because lead balls operate differently chemically, but they would still react to each other in a predicatable way as far as gravity was concerned.

There seems to be uniform within our galaxy/universe.

There is, which is helpful for understanding it, if different bits operated differently, well then we'd really be in trouble because nothing would be predictable.

6. [Light travels at c within a vacum, however our universe seems to have a force all of it's own, how do we know that what we see is correct to our calculations of c as we know it ?

I'm not quite sure what you're after here.

What I meant, was. Does light travel as quickly through gases or liquids as it does in a vacum ?

7. Originally Posted by Sean Clayden
[Snip!] What I meant, was: Does light travel as quickly through gases or liquids as it does in a vacuum?
It travels more slowly through solids, liquids, and gases. For example, the speed of light in water is roughly (2/3)c.

8. What I meant, was. Does light travel as quickly through gases or liquids as it does in a vacum ?

It travels more slowly through solids, liquids, and gases. For example, the speed of light in water is roughly (2/3)c.

While what CM says is true, it's also not quite as easy as that. I don't want this turning into a "properties of light" thread, and I'm sure there are a few about is you want to get into it, but the real answer is that light always travels at c, however when you put it into a different medium (such as water or glass) it appears to slow down due to being continually absorbed and retransmitted by the atoms inside the medium as it passes through. The photons themselves are always travelling at c however.

Imagine it like this. You want to drive 100km. You can only drive it at a constant speed of 100km per hour. If you drive straight there it'll take you an hour, your average speed being 100km/h. If you stop every 15 minutes of driving for a 5 minute tea break then you'll take 1 hour and 15 minutes, so your average speed is only 80km/h even though your car can only travel at 100 km/h when it's moving.

9. Originally Posted by PhantomWolf
What I meant, was. Does light travel as quickly through gases or liquids as it does in a vacum ?

It travels more slowly through solids, liquids, and gases. For example, the speed of light in water is roughly (2/3)c.

While what CM says is true, it's also not quite as easy as that. I don't want this turning into a "properties of light" thread, and I'm sure there are a few about is you want to get into it, but the real answer is that light always travels at c, however when you put it into a different medium (such as water or glass) it appears to slow down due to being continually absorbed and retransmitted by the atoms inside the medium as it passes through. The photons themselves are always travelling at c however. [Snip!]
Exactly right, I should have been clearer.

10. I should have been clearer.

eeeh, for most people your answer would have been enough, I just felt that Sean was seeking something deeper so I wanted to make sure that he got it.

11. Originally Posted by PhantomWolf
the real answer is that light always travels at c, however when you put it into a different medium (such as water or glass) it appears to slow down due to being continually absorbed and retransmitted by the atoms inside the medium as it passes through.
I dunno if that qualifies as the real answer. The individual photons are not absorbed and retransmitted as they pass through transparent mediums.

12. I got it thanks. A point I have to then make is that the calculations of distances from light sources that we see would then only be estimates/approximations, the light/photons could have been reduced by passing through a multitude of gas clouds etc before reaching us.

13. Originally Posted by hhEb09'1
You feel the effect of gravity on the rest of your body as well. Lift your arm up, it has weight--if you doubt it, do it 2000 times and see if you get tired

Sometimes my head seems heavier too, like now
It would get tired in zero gravity too......

14. "It" meaning "you" or "my head"?

OK, just hold your arm out-stretched for an hour, you'd get tired. In zero-gravity, not so much.

15. I got it thanks. A point I have to then make is that the calculations of distances from light sources that we see would then only be estimates/approximations, the light/photons could have been reduced by passing through a multitude of gas clouds etc before reaching us.

The speed it travels isn't what we use to it though. We use several, the major two being Parallax caused by the Earth's orbit about the Sun and the Redshift of the light's spectrum. These are unaffected by changes in the light's speed through a medium.

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Originally Posted by Sean Clayden
Does gravity push or pull?
It sucks!

17. Originally Posted by SillyMidOff
It sucks!
Somebody pointed that out earlier so that is two votes for "it sucks". We may have a winner!

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