Gravity how is it formed

[metz]

Earths Gravity is strong enuff to have a tremendous pull on the moon , whereas down on little old earth its very weak....in short where does it all come from and why is it so weak here on earth ?

[WayBig]

waek? have you ever tried to jump off the earth? how far did you get?

[ollypenrice]

Gravity is associated with mass and energy. Where there is either there is gravity. As far as I know, why this is so is not known.

Massive objects (objects with mass) have a centre of gravity from which their gravitational field extends. The strength of this gravitational field, as discovered by Newton, falls off at the inverse square of the distance from the centre of gravity. This inverse square law simply means that the gravitational field will be four times less if you double your distance from the centre of gravity. Quadruple the distance and the field drops to sixteen times less, and so on.

The Earth and the Moon in fact pull on each other. If you put them on a see-saw they would balance at a point so close to the earth as to lie inside the earth's radius because the earth has so much more mass. It is larger and much denser than the moon. They both orbit around this imaginary point of balance, so it is not strictly true that the moon orbits the earth.

It is not always helpful to think of gravity as a force. In General Relativity it thought of as a curving of space and time around an object with mass.

WayBig makes a good point. You need to be blasted up to about 25,000 mph if you want to escape the earth's gravitational field and not fall back down again, which is why it took us a while to find out how to do it!

Olly

[Tarazed]

To follow Olly's excellent post above the point at which the Earth and Moon orbit each other is called the 'barycenter'.

The most recent and widely accepted theory regarding Gravity is Einstein's theory of general relativity. Building upon Newton's law which says that objects attract with a force proportional to their mass Einstein's theory proposes that objects with mass create a curvature in 'spacetime' and it is this curvature that we experience as Gravity.

If you are a fan of Professor Brian Cox his book 'Why Does E=MC2' provides informative and accessible reading covering topics such as the universal speed limit of 'light speed' and the 'warping of spacetime' (ie Gravity).

[Zakalwe]

Earths Gravity is strong enuff to have a tremendous pull on the moon , whereas down on little old earth its very weak....in short where does it all come from and why is it so weak here on earth ?

On the contrary, the pull of the Earth's gravity is stronger on Earth than it is at the moon. The strength of gravity changes inversely as the square of the distance (if you double the distance then the gravitational field is four times weaker). So the Moon experiences a much lower gravitational pull from the Earth than you do on the surface of the Earth.

<edit> What is true, is that gravity is a very weak force, many, many times weaker than the the other three fundemental forces.

What is different is that the strength of these much stronger forces are inversely proportional to the cube of the distance, i.e. if you double the distance then the effect is 8 times weaker. This explains why these forces only really act within the boundaries of an atom...their effect declines far too quickly as you move outside an atom. Gravity, although much much weaker doesn't decline as quickly, hence bodies in the universe don't fly apart*

*yes, yes, I know...

[JamesF]

I've just realised! This means that if you're on a diet, it's better to weigh yourself when the moon is high in the sky!

James

[smoggy]

I've just realised! This means that if you're on a diet, it's better to weigh yourself when the moon is high in the sky!

James

Apart from when you weigh yourself in kg or stone as most of us do then that would actually be your mass so will not change

[Earl]

Gravity although we think of it as strong a simple weak magnet can interfer with it.

[JamesF]

Apart from when you weigh yourself in kg or stone as most of us do then that would actually be your mass so will not change

You've lost me there.

If you're weighing something, whatever the units, you're effectively measuring the effect of gravity on its mass. You can weigh in it stones, kilograms or feathers, but you're still measuring the same thing. And, if the moon is overhead it will have the effect of cancelling out a tiny little bit of the earth's gravitational effect on your mass so you'll weigh (a tiny tiny bit less. If the moon is around the other side of the planet then it will add to the gravitational "pull" of the earth and make you weigh more.

That your mass won't change is certainly true, but you can't generally measure mass by weighing things at all. We've just fudged things so the two are interchangeable at one Earth gravity for everyday purposes.

If you always weigh the same if you measure in kg, then what would happen if you stood on a set of kg scales on the moon?

James

[smoggy]

If you always weigh the same if you measure in kg, then what would happen if you stood on a set of kg scales on the moon?

James

Your mass would be the same (around 70-80kgs for the average man) but your weight would be about 5-6x less.

Weight is measured in Newtons, mass is measured in kgs, stone, lbs etc.

Here on Earth we use the two interchangeably but a physicist will make the distinction between the two. Weight is the force due to gravity on a mass.

[JamesF]

Your mass would be the same (around 70-80kgs for the average man) but your weight would be about 5-6x less.

Weight is measured in Newtons, mass is measured in kgs, stone, lbs etc.

Here on Earth we use the two interchangeably but a physicist will make the distinction between the two. Weight is the force due to gravity on a mass.

Exactly. So I'm still at a loss as to the meaning of your comment:

Apart from when you weigh yourself in kg or stone as most of us do then that would actually be your mass

but I think it's clear that we both know what we're talking about in the general scheme of things, so never mind.

James

[Shibby]

If you always weigh the same if you measure in kg, then what would happen if you stood on a set of kg scales on the moon?

Everyone knows that a proper pair of moon scales is calibrated to the moon's gravity. You should therefore see the same mass on the read-out. How would we measure our flour correctly when baking otherwise?? You'd end up with an uber-cupcake nearly 6 times the size!

[JamesF]

You'd end up with an uber-cupcake nearly 6 times the size!

A plan with no drawbacks

James

[ollypenrice]

A plan with no drawbacks

James

...bar an regrettable effect on your weight.

If we get to travel regularly in space I daresay that there will be a new organisation called Masswatchers...

Olly

[Photosbykev]

...bar an regrettable effect on your weight.

If we get to travel regularly in space I daresay that there will be a new organisation called Masswatchers...

Olly

but you are six times lighter to start with so you can eat more

[lguise]

This is the sort of thread I love on a wet Tuesday afternoon!!

But accepting that Gravity is the weakest of the forces, follows the inverse square law, and that it is best thought of as bent space time, I still don't think we know what causes it!

I teach Physics, all be it to a fairly humble level, but children always hit you with that question. My stock answer is that the fact that we don't know is exactly why we have to fund science and do great experiments such as the LHC. That normally buys me some time till the next question I can't answer, which is usually something along the lines of what happens inside a Black Hole?

I can't get the idea of curved space around a very large cupcake out of my mind at present...starting to feel a little peckish!

Linton

[Si W]

I find that Gravity usually kicks in around about 12 pint, the floor seems to come from no where .

[metz]

Gravity is associated with mass and energy. Where there is either there is gravity. As far as I know, why this is so is not known.

Olly

Why this is so is not known ...thats the question ...nice reply tho olly good reading thankyou