# Why does the earth and the sun and other planets have gravity?

Vorbis
This might sound like an unoriginal question but what is gravity? Why does the Earth and the sun and other planets have gravity? Does everything have gravity? What causes gravity?

fatra2

Mass causes gravity field, just like electric charge causes electromagnetic field.

Vorbis

Well everything has mass so does that mean everything has its own gravity? How come that if I put a marble on a table it doesn't role towards me?

Mentor

Why does the Earth and the sun and other planets have gravity?

Physics can never give the complete answer to a deep "why?" question like this one. Any answer always leads to another "why?" question.

How far do you want to go? Newton's law of gravitation? The Einstein field equations (general relativity)? String theory (very speculative at the moment)?

Bob_for_short

Well everything has mass so does that mean everything has its own gravity? How come that if I put a marble on a table it doesn't role towards me?

The friction force is more strong than the gravity force in your case, that is why. H. Cavendish measured attraction of masses in a more clean experiment.

Bob.

Vorbis

The friction force is more strong than the gravity force in your case, that is why. H. Cavendish measured attraction of masses in a more clean experiment.

Bob.

So The marble would role if the surface was extremely, impossibly smooth?

Buckleymanor

So The marble would role if the surface was extremely, impossibly smooth?
No.
You just ain't big enough in comparison to the Earth for the marble to be atracted to you.
The marble could be suspended on the end of a string to overcome most frictional forces but it's deflection towards you would be so minimal it would be very difficult to measure.
The Cavendish experiment used a mountain and some very sensitively mounted weights to measure the attraction of mass, and if memory serves me right it took quite a long time to set up the equipment.
Even then the deflection was minimal and had to be observed via a telescope I think.

Vorbis

Hmmm... So if I made a brick (or something) the size of earth, would it attract comets and stuff?

frankinstein

This might sound like an unoriginal question but what is gravity? Why does the Earth and the sun and other planets have gravity? Does everything have gravity? What causes gravity?

Let's see apparently nobody knows...some have said that gravitons exist, but no one has detected an actual graviton, However fields in general have common characteristics, such as they all behave under a radiated scheme where the distance from the source of a field dilutes its strength, this is known as inverse square law, they also can invoke positive or negative momentum and radiate at the speed of light. Light also is bounded by an inverse square law, can impose momentum and radiates at the speed of light...If it quacks like a duck and acts like a duck then its a duck. Unless of course its a parrot that can sound like a duck, but then ducks and parrots are both birds...

Frank

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No.
You just ain't big enough in comparison to the Earth for the marble to be atracted to you..

No, that's not right. The presence of the Earth has no effect on the force between you & your marble. The force is much smaller than the force between the Earth & the marble, but that's not the point. The OP is right, there is a force between him & his marble, and the Cavendish experiment is a clever way of measuring it.

The marble could be suspended on the end of a string to overcome most frictional forces but it's deflection towards you would be so minimal it would be very difficult to measure.

Difficult but not impossible. In general, that's pretty close to what Cavendish did.

The Cavendish experiment used a mountain and some very sensitively mounted weights to measure the attraction of mass, and if memory serves me right it took quite a long time to set up the equipment.
Even then the deflection was minimal and had to be observed via a telescope I think.

The Cavendish experiment is routinely done in physics classes. It is done by measuring the attraction between two sets of weights, one set is large and the other is small. No mountains are involved. The weights are arranged in a torsional balance, what gets measured is the 'twist' induced by the gravitational force (and counteracted by the torsion of the twisting wire). Check google or wiki for a more detailed explanation with some graphics to make the setup more clear.

As far as the mountain goes, Cavendish used the result to calculate the mass & density of the earth. Maybe that's what you're trying to remember.

Vorbis

If I weighed, let's say 10,000,000 tons, would the marble be attracted to me or am I just a confused child?

The marble will always be attracted to you, but if you weighed a lot more, it would be a lot easier to measure.

Vorbis

Oh that makes sense

fatra2

Hi there,

When you talk about the Earth and the Sun's gravity pull, you have to understand Newton's third law. If the Earth pulls on you, you pull on the Earth. Therefore, you develop an gravitational pull on the objects around you.

cragar

Mass causes gravity field

mass or energy causes a gravitational field , like light .

fatra2

Hi there,

The duality between energy and mass, from Schrödinger's equation, can also be interpreted in the way that light reacts to the gravity pull of objects.

Buckleymanor

No, that's not right. The presence of the Earth has no effect on the force between you & your marble. The force is much smaller than the force between the Earth & the marble, but that's not the point. The OP is right, there is a force between him & his marble, and the Cavendish experiment is a clever way of measuring it.

Difficult but not impossible. In general, that's pretty close to what Cavendish did.

The Cavendish experiment is routinely done in physics classes. It is done by measuring the attraction between two sets of weights, one set is large and the other is small. No mountains are involved. The weights are arranged in a torsional balance, what gets measured is the 'twist' induced by the gravitational force (and counteracted by the torsion of the twisting wire). Check google or wiki for a more detailed explanation with some graphics to make the setup more clear.

As far as the mountain goes, Cavendish used the result to calculate the mass & density of the earth. Maybe that's what you're trying to remember.

Cavendish, contrary to what many physicists think, did not follow this procedure. Cavendish simply computed the density of the Earth without the intermediate step of finding the value of G. The value of the universal gravitational constant, G, was not computed for roughly a century after Cavendish did his experiment.

Read more: "Understanding the Cavendish Experiment: Measuring Newton's Universal Gravitational Constant, G | Suite101.com" - http://physics-history.suite101.com...ding_the_cavendish_experiment#ixzz0FwaFnNVe&A

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Vorbis

Hi there,

When you talk about the Earth and the Sun's gravity pull, you have to understand Newton's third law. If the Earth pulls on you, you pull on the Earth. Therefore, you develop an gravitational pull on the objects around you.

So I have gravity because the Earth has gravity or do we both allready have gravity and we are pulling on each other?

cragar

you both already have a gravitational field.

fatra2

Hi Vorbis,

You are exactly right. From Newton's third law, the Earth develops a gravitational pull on you, but you do the same on the Earth. You can extend this idea, to the next lady you meet in a bar. You can go and say that the laws of nature make you attract each other.

Ok, the pick up line is pretty raunchy, but coming from scientist it half funny.

Fact is that you attract every object around you. The total interaction depends greatly on the distance to the object, but in a very general sense, you develop a gravitational pull on every piece of object in this Universe. Sweet no??

Cheers

Gold Member

If I weighed, let's say 10,000,000 tons, would the marble be attracted to me or am I just a confused child?
Let's just make it clear: not only is the marble attracted to you by thee gravitational pull exerted by your ~75kg mass, but you are attracted to the marble by its ~5g mass.

These forces are easlily calculable using the standard gravity formula F=G m / (d^2) and is commonly done as part of many a physics homework assignment.

However, the pull of your mass on the marble at a few metres distance easily exceeds the pull of Mars or other planets on the marble.

Vorbis

Thank you all so much for the replys I'm motavated about physics. You people inspire me