Gravity - Has anyone figured it out...?

[Turbofan]

OK, so the earth spins and we get sucked to it (gravity), why???

What is it that makes us stick to the ground???

(And don't answer simply gravity, I'm after a more detailed explanation, if there is one...)

Cheers,

Turbofan

[compressor stall]

If the earth stopped spinning you would find getting out of bed rather difficult indeed.

*rummages through the depths of his brain for the formula for the gravitaional attraction of 2 bodies but can't find it*

Being a pedantic sematic, gravity does not suck either - it is a force between 2 bodies of any size. Just it ain't that noticable unless you are talking about items of Beasleyesqe proportions.

[gaunty]

The fact that the earth spins and exerts a gravitional influence have nothing to do with each other.
Even if it wasn't spinning the gravitational force would remain the same.
Gravity as far as I can recall just "is" a property of matter. ie all matter posseses it but its influence is relative to the mass on an inverse square law relative to the distance between them. The closer they are to each others centre of mass, the stronger the influence.
If there is a simple explanation it can be found in books such as Hawkings "Brief History of Time" which goes a way to explaining the various "weak" and "strong" forces in nature. If I recall correctly gravity is a relatively "weak" force. Our personal mass relevant to the mass of the earth for example is what stops us floating away. The further we get away from earth as a massive mass the less influence it has and the more our mutual personal gravitational forces would come into effect. Hence the desire to take our amour to the moon.
The whole universe is a matrix of interacting gravitational forces.
Exactly what it "is" as far as I can find out is still one of the "mysteries".
As soon as they find out we had better be ready for retraining soonest.
Thus the observation that one "has a magnetic personality" may or may not be a confusion of the physical laws of electromagnetism and gravity.
They are all interelated and we are still searching for the Universal Law that connnects them all.
A fascinating and rewarding subject that when it gets down to the quantum level like Alice says "gets curiouser and curiouser" the more we learn and find out.

Slasher where are you?

[Dave Incognito]

The earth’s rotation doesn’t play a huge part in affecting the gravitational force we experience. Assuming the world was perfectly spherical, by my humble calculations, the difference between the acceleration due to gravity at the poles and the equator should only be around 0.03m/s^2.

I am lead to believe that the actual difference is also affected by the earth’s bulge at the equator. Apparently this increases the distance between the centre of mass of the two bodies (i.e., you and the earth) which results in a lower acceleration due to gravity value at the equator.

Like compressor stall and gaunty said, gravity is an attracting force between two bodies with the magnitude of the force depending upon the mass and distance between the bodies. The force decreases/increase in proportion to the square of the distance, i.e. 1/(distance)^2.

Have a good one, Dave.

[ 16 August 2001: Message edited by: Dave Incognito ]

[compressor stall]

Yes dave you are right - people are not substantially heavier at the poles.(where there is less centripetal force from the earth;s rotation).

[Hermie]

Gravity (or more properly, gravitation) is the attraction which all particles of matter exert on one another.

Newton's model says that every particle in the universe attracts every other particle with a force which is directly proportional to the mass of each particle and which varies inversely as the square of the distance between them:

F = G.m1.m2
___________

r2

Here m1 and m2 are the masses, r is the distance between them and G is a fixed quantity called the Universal Gravitational Constant.

---------------------

All this means is that if you double either of the masses, the attractive force will double, but if you double the separation, the attractive force will go down to one quarter (1/22) of the original value. G has to be found by experiment; it is very small ( 0.000 000 000 067 newton metre2 / kilogram2 ), showing that gravitation is a very weak force.

Albert Einstein produced a different model of gravitation. He did away with forces altogether ! Instead, he said that any body possessing mass would produce a distortion in space and time. Empty space, according to Einstein is 'flat' and in this kind of space, bodies travel in straight lines. Space containing bodies having mass is 'warped' so that the most natural path changes from being straight to being curved. On this view, the Moon is not held in its orbit by gravitational forces; it merely follows a natural path in the curved space created by the presence of the Earth.

Hope this helps

Cheers,
Herman

[Hermie]

Hi again,

The Earth is not quite a perfect sphere. Because it spins faster at the Equator than at the poles, Earth bulges at the Equator. Scientist describes Earth's shape as 'geoid', which simply means Earth-shaped !

The reason earth spins is rather simple. It is because, it is falling around the Sun. As the Earth hurtles round the Sun, the Sun's gravity keeps it spinning, just as the Earth's gravity keeps a ball rolling downhill.

Well sure hope this helps...

Best Regards,
Herman

[ft]

The reason earth spins is rather simple. It is because, it is falling around the Sun. As the Earth hurtles round the Sun, the Sun's gravity keeps it spinning, just as the Earth's gravity keeps a ball rolling downhill.

Ehm, no. It spins due to rotational inertia - and the tides are slowing it down! Yes, the days are getting longer (most of that time seems to be added around 16.30, don't you think?). As a matter of fact, I seem to recall that when human beings are completely isolated from outside time references, we fall into 23 hour sleep patterns so we're not completely keeping up with the world biologically.

Cheers,
/ft

(A longer explanation)

(Edited to add link)

[ 16 August 2001: Message edited by: ft ]

[twistedenginestarter]

You stick to the ground because you are exchanging particles called gravitons with the earth beneath you. In fact you're exchanging gravitons with all the other matter in the universe but the other bits have minimal impact by comparison. Yeah-right! Our little planet versus the entire Universe - doesn't add up does it?

There's another problem. Presumably as the number of aircraft flying increases there will be an ever increasing shortage of gravitons to service them all. You'll notice this if you get to TOD, idle the engines and then... 0 fpm on the VSI.

However, fear not.This cannot persist for very long because queuing superstring theory tells us that after a short pause a bunch of gravitons will arrive together like No 49 buses.

Which, of course, explains Clear Air Turbulence, so always keep your seat belt nicely tightened.

[VivaTheBeaver]

I was thinking of starting a focus group to mull over the merits of making gravity illegal. You see I think Tony would really go for this as the next level in the UK nanny state. The rational would be that gravity is causing millions of man days off sick each year all due to gravity related incidents, such as trips and falls. I also think that the Insurance companies would like to jump on this band wagon so that you will no longer be able to make a claim for falling off a ladder as it would be classed an act of God (Or isn’t Tony that powerful yet ?) If Tony can’t sort this out I’m moving to the moon, where it’s free from gravity related incidents.

VTB

[tony draper]

Personelly I think we have it the wrong way round,all matter repels and and the force of repulsion increases with distance,hence the large universe out there pushes us down here on Earth, I think all the sums will still work, must rush off a paper to New Scientist
Didn't Dirac have something to say about rotating bodies and gravity?.

[FlyFreeWbe]

No People at the poles are not heavier because of the lack of the centripetal force. You're getting confused between weight (Kg) and mass (N). If you could all remember back to GCSE school years, you'd know that f=1/d showing that force is inversly proportional to distance. That only affects mass.
If you had an astronaut in space, he wouldn't weigh less, but have a different mass. (Bearing in mind that mass is the force of gravity acting on an object or particle) Remember Haley's (not sure of spelling) comet? As it goes around the sun it speeds up. All must remember Armageddon (the film) where they go around the moon and zoom of at how many thousands of kmph! It's the same here, the closer you are to an object having a force proportional to its mass, the more that force will act on you. Same as a high G turn/climb/whatever..

Sorted!
Surprised how much I remember from science lessons

[ 16 August 2001: Message edited by: FlyFreeWbe ]

[FlyFreeWbe]

<shorter answer>...Gravity is just something thats there. Just accept it and don't go jumping off cliffs.
If you really need to know what it is, then maybe you're looking for too much in life. Even the best scientist doesn't know everything!!

[ft]

Actually...

1) The acceleration needed to remain on the surface in spite of the earth's rotation at the equator would amount to about 0.037 m/s^2 or 0.004g according to quick and possibly incorrect calculations (a=v^2/r, d=14*10^6 m, feel free to doublecheck) so there IS a difference albeit very much a negligible one, especially as the gravitational constant varies approximately between 9.80 to 9.82 depending on latitude anyway IIRC.

2) The gravitational force is inversely proportional to the square of the distance. F~1/r^2.

3) Mass remains constant (except in nuclear reactions). Weight is the force the scale measures and changes with the gravitational constant etc.

Cheers,
/ft, certified nitpicker

(Edited to make sense)

[ 16 August 2001: Message edited by: ft ]

[kabz]

Whatever happened to that chap in Dundee who was on the news working on an anti-gravity machine.

I seem to remember lots of flywheels and stuff and him showing that the thing actually got lighter, when running.

Visions of Dundonian UFOs etc etc...

It was on Reporting Scotland, so there must be **something** to it.

[ft]

kabz: He floated off, never to be seen again?

Cheers,
/ft

[Checkboard]

I know we all like these discussions, but the strength of PPRuNe is that it is about matters aviation, rather than maintaining freindships across the world on matters interesting to technically minded people (including me! )

There is a wealth of serious information on physics on the web - have a look for that. I am going to close this, and all related threads on that basis. Sorry