gravity energy

[megaflus]

I was thinking about something the other day, a planets gravity is energy and it's because of the size of the planet you know. but there can't be unlimited energy and our energy comes from the sun then what happens if there is no sun then there is no energy that means that there shoulden't be any gravity but there is (I think) that means it is unlimited energy. do you know what I mean. explane

[chris_uk_83]

Right, time for a short lesson on gravity. I'll try to address all your questions the best I can. Then follow up with a brief introduction to the intricacies of gravitational fields.

a planets gravity is energy

Not quite. You need energy to move further away from the origin of the graviational field and you gain energy by moving towards the origin of the gravitational field, but it isn't correct to say that "gravity is energy".

it's because of the size of the planet

Again no. It's because of the mass. This usually relates to size so I can see where the confusion arises, but a more dense body (e.g. a neutron star) will have a higher gravitational field than a similarly sized planet.

but there can't be unlimited energy

True.

our energy comes from the sun

Yes, this is true. Energy arrives on Earth from the sun by means of electromagnetic (EM) radiation. This can be light (visible), heat (infra red), or other radiation (ultraviolet, x-rays, gamma, radio and microwaves). Most of the radiation that is harmful to life is screened out by our atmosphere.

We can utilise the heat and light ourselves, or indirectly through plants; which are awesome at turning light into more plant (ask Fish how this works, I'm no biologist).

The energy that comes from the sun arises as a result of a nuclear fusion reaction going on inside the sun, not from its gravity. Gravitational force keeps the Earth from flying away from the sun, but we can't take any gravitational energy from the sun unless we move closer to it.

what happens if there is no sun then there is no energy that means that there shouldn't be any gravity but there is

Again no. You take away the sun and you take away the heat and light that the sun provides. You also take away the force that is keeping the Earth in orbit, so Earth would 'fly off'. Earth, however, has mass and therefore will have its own gravitational field regardless of any input the sun may provide. Nothing, apart from the Earth itself, is responsible for the gravity we feel while stood on the Earth (technically there's more to it than that but don't worry about it too much).

explane

I will explain.

Gravity is not a 'thing' as such, but a property attributable to a body. Everything that has mass has a gravitational field (including you), though the force due to your gravitational field would be very small (smaller than a gnat's fart) since your mass isn't that big.

You can work out the gravitational force between two objects using the following simple bit of maths:

F = GMm/r²

where F is the attractive force between the two bodies, G is the gravitational constant (6.67 x 10^-11Nm²kg^-2), M is the mass of body 1, m is the mass of body 2 and r is the distance between them.

This means that only two things that have mass will be attracted to each other (since if M or m is 0 F becomes zero).

I recommend trying this calculation out a little and putting different numbers into it. For instance try your mass and the mass of the Earth (which you can easily Google), r in this case will be the radius of the Earth (which you can also Google). This will give you the force you feel in Newtons, divide this by 9.81 and you ought to get your mass back in kg.

Any more gravity related questions, don't hesitate to ask

[Curtana]

Very good and clear explanation. Well done.


All right genius I have related questions because I am shameless of my ignorance.
There are 4 fundamental forces in the universe: strong, weak, EM and gravity.
As I understand it two objects can interact magnetically or electrically because the field 'tells' them the other one is there.
Question 1. Is the field made of photons? (Q 1 parte deux) If it isn't what is it?
Question 2. Following on from above is gravity transmitted through gravitons?
Question 3. Is anti-gravity theoretically possible?

[ajm317]

Question 1. Is the field made of photons? (Q 1 parte deux) If it isn't what is it?

Yes, it is. These are known as "virtual photons". The electromagnetic interaction between a positive and negative particle can be explained through the exchange of these photons.

This is known as Quantum Electro-Dynamics (QED) and it is...complicated. Very complicated. Complicated enough so that I can't explain it much beyond what I already have. That said, it is fully understood by scientists and there are all sorts of amazing experimental results that match theory to the 13th decimal place or whatever.

[Noble Savage]

(smaller than a gnat's fart)

Is this a technical term ?


Thanks for the lesson , Couldn't remember the theory behind it all (was 17 years since I was last in a physics lab)

[Gram -ve]

Kind of amazing when you think that every particle with mass in the universe exerts gravity on everything else (even if it is less than negligible).

[Simetrical]

I was thinking about something the other day, a planets gravity is energy and it's because of the size of the planet you know. but there can't be unlimited energy and our energy comes from the sun then what happens if there is no sun then there is no energy that means that there shoulden't be any gravity but there is (I think) that means it is unlimited energy. do you know what I mean. explane

The important thing to realize here is that energy is only required to do what's technically called "work". Energy is not necessary required to move stuff around. It's a funny quirk, but true. One important point is that it takes no energy to exert a force perpendicular to the direction of motion. This means that, for instance, the floor does not expend energy to keep you from falling through it (assuming it doesn't bend when you step on it). This is why a statue can stand where it is forever without falling to bits, too: work is not done on stationary objects, so energy will not run out.

But by the same token, the Sun does no (net) work on the Earth. The gravity of the Sun pulls the Earth toward the Sun, but the Earth is actually moving at almost exactly a right angle to the force, so the Sun does virtually no work. This makes the configuration stable, so as long as the Sun exists and doesn't lose mass and there are no other major forces, the Earth will just orbit the Sun forever and ever. Likewise satellites require only small amounts of energy to adjust their course once they're in orbit: the orbiting comes free, since nothing is exerting a significant force in the same direction as the satellite is moving.

There are 4 fundamental forces in the universe: strong, weak, EM and gravity.
As I understand it two objects can interact magnetically or electrically because the field 'tells' them the other one is there.
Question 1. Is the field made of photons? (Q 1 parte deux) If it isn't what is it?

Classically, the field was just viewed as an abstraction with no real substance. Basically, people just used "the electric field at point X has a magnitude Y newtons per coulomb in direction Z" to mean "if I stuck a small charge, say a nanocoulomb, at point X, a force of Y nanonewtons would be exerted on it in direction Z". It was a mathematical description used for convenience.

Nowadays, in particle physics, photons are viewed as the carriers of the electromagnetic force. You could thus, I suppose, define the electric field at a point in terms of the density and characteristics of photons in that region.

Question 2. Following on from above is gravity transmitted through gravitons?

Maybe. Gravity is not part of the standard model of particle physics. The current authoritative model for gravity, general relativity, does not countenance quantized particles at all, but rather continuous spacetime. The unification of these two viewpoints is the major topic of theoretical physics right now, and various hypotheses that seek to unify them have introduced particle carriers of the gravitational force. Gravitons have not been observed, and their existence would be a major breakthrough in physics.

Question 3. Is anti-gravity theoretically possible?

No. Or rather, there's no particular reason theorists see to think it's possible, and certainly no evidence suggests it is in any common sense. Of course, anything's possible ultimately.

[David Deas]

Energy is not necessary required to move stuff around.

Thats not true. In order to move an object you must give energy to that object. There are no exceptions.

[Thanatos]

Actually, in the end, nobody really knows.

All science says is "That's how it's observed to do so."

They can't explain the "Why."

[Valentin the II]

Actually, in the end, nobody really knows.

All science says is "That's how it's observed to do so."

They can't explain the "Why."

So, whats your point?
Yes we observe things and with theories we explain the "why" (or atleast try to).

If you dont have anithing meaningfull to say, then go argue about gods and heroes somwhere else and let the smart ones do the talking.

Question: Whats the bases (evidance) for the anti-matter/black matter theory?
I never figured out how they came up with this thing.

PS: sory for the shameles rant, just expresing my feelings.

[chris_uk_83]

Thats not true. In order to move an object you must give energy to that object. There are no exceptions

Yes there are and gravity is one of them.

You must expend energy to change the speed of an object, but not necessarily to change its direction. If you had a game of swing ball (a tennis ball on a string on a post) in zero gravity with no air resistance and you hit the ball it will go around the pole at a constant speed (neglecting friction). It will constantly change direction though because the string is exerting a force at 90^o to the direction of motion. The string, though applying a force is not transferring any energy to the ball.

Gravity is basically a big string.

Is this a technical term ?

Yes, of course it is!

[Curtana]

Thank you simetrical. If only my physics teachers had been so articulate.

[Gary88]

That said, it is fully understood by scientists and there are all sorts of amazing experimental results that match theory to the 13th decimal place or whatever.

QED is accurate to the point that if you were measuring the width of th USA to the same accuarcy you would be accurate to within the width of a hair.

[ajm317]

QED is accurate to the point that if you were measuring the width of th USA to the same accuarcy you would be accurate to within the width of a hair.

Like I said, accurate.

Question: Whats the bases (evidance) for the anti-matter/black matter theory?
I never figured out how they came up with this thing.

Which do you mean? Anti-matter? Or dark matter?

[chris_uk_83]

Dark matter is something to do with cosmology. Observed gravitational effects (e.g. looking at the way galaxies move) don't quite fit the model of gravitation we currently have. Cosmologists figured that if there was a load of extra mass then the movement could be explained with our model of gravitation. They never found the mass so they dubbed it "dark matter", becasue it can't be seen.

To me (a proper experimental physicist) this looks like a bodge job. However, it's the accepted theory at the moment and I can't come up with anything better

Dark energy is something different again.

Antimatter is just the opposite of matter. Because of some complexities of particle physics (which I doubt you want me to go in to) every particle has its "anti" particle. If these two meet, they annihilate and release energy. These have been experimentally observed in particle accelerators (again, I could explain if you want) and are quite real. The question that stumps the physics community at the moment is "why is there more matter than anitmatter?" Since both ought to have been created in equal quantities in the big bang.

[Curtana]

When they say 'there is nothing that can't be solved by a nice cup of tea' they weren't being metaphorical!

http://www.telegraph.co.uk/connected...21/ectea21.xml

Sorry but making that link was bigger than me, you, TWC, maybe EVERYTHING.

[chris_uk_83]

There's already been a thread on that somewhere, and what have disappearing teaspoons got to do with gravity?

[Curtana]

They are one postulated explanation for the missing 22% of matter in the universe. This is vaguely related to Valentin's q. It also explains that unexplained gravitation you were wotting whereof howsofor. I think you're taking one minor thoughtless post a bit seriously there. It is a theory. It is a ridiculous and largely unfounded theory but hey, have you read any string theory recently?!

[chris_uk_83]

Fair enough then, as long as teaspoons are actually dark matter To be honest, I think teaspoons being responsible for most of the unexplained effects in the universe is far more convincing than string theory.