# The Naked Scientists Forum

### Author Topic: a question of gravity  (Read 2880 times)

#### barneyboy

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##### a question of gravity
« on: 23/10/2014 22:57:41 »
does gravity increase with the mass of an object?

#### barneyboy

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##### Re: a question of gravity
« Reply #1 on: 23/10/2014 23:30:54 »
and if every action has an equal but opposite reaction, what is the opposite reaction to gravity?

#### chiralSPO

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##### Re: a question of gravity
« Reply #2 on: 24/10/2014 00:53:27 »
The strength of an object's gravitational field does increase with how massive the object is. This means that a more massive object will exert a stronger gravitational pull on other objects, and it will also be pulled on more, that is the opposite reaction: both objects fall towards each other, and the tug of one object on the other is equal to the tug going the other way.

#### PmbPhy

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##### Re: a question of gravity
« Reply #3 on: 24/10/2014 04:55:26 »
Quote from: barneyboy
does gravity increase with the mass of an object?
As chiralSPO explained, yes. In addition to that there are two additional sources of gravity, namely stress and momentum. If you know advanced calculus and differential geometry and tensor analysis then you'd know what a tensor was.  In a way they can be thought of as matrices. But they're special tensors. The components must obey a certain rule when you transform from one set of coordinates to another.

There are various kinds of tensors. One is called the Stress-Energy-Momentum Tensor. It's defined here:
http://home.comcast.net/~peter.m.brown/sr/energy_momentum_tensor.htm

The components are stress, energy and momentum. Mass comes into play through the mass-energy relationship E = mc2. I actually think of it as the stress-mass-momentum tensor. I say this because mass is a physical quantity whereas energy isn't. Energy isn't a real physical quantity. It's merely a way to keep track of certain relationships between physical quantities called "energy" where mass is more of a physical quantity in that it's the property of matter to resist acceleration. However you should keep in mind that both stress and momentum contribute a very very very small quantity to the strength of the gravitational field.

So in general relativity the strength of a gravitational field is a function of mass, stress and momentum. You can read more about this on my webpage at http://home.comcast.net/~peter.m.brown/gr/gr.htm

#### evan_au

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##### Re: a question of gravity
« Reply #4 on: 24/10/2014 22:37:35 »
Quote
does gravity increase with the mass of an object?
Yes, but...
• Gravitational forces also decreases with the square of the distance between the objects (this is called the inverse square law).
• In particular, the surface gravity decreases as the square of the radius (for a given mass).
• So if you compare the surface gravity of a dense asteroid (made mostly of iron, for example) with the surface gravity of a comet with twice the mass (made mostly of water, for example), then the surface gravity of the comet would be lower.

#### barneyboy

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##### Re: a question of gravity
« Reply #5 on: 25/10/2014 13:29:54 »
so at some point in every mass there would be (near) zero gravity as there would be equal mass and no distance on all sides and little or no centrifugal force.

#### evan_au

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##### Re: a question of gravity
« Reply #6 on: 27/10/2014 20:05:18 »
Yes, at the centre of a spherical planet or asteroid, you would float "weightless", as the gravitational attraction of mass over your head is cancelled by the mass under your feet.

Of course, there are some practical difficulties in digging to the centre of the Earth or an asteroid.

In practice, it is easier to get into orbit than it is to get to the centre of the Earth. In this case, there is still a gravitational attraction, but you are falling under the influence of gravity, so you feel weightless.

#### barneyboy

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##### Re: a question of gravity
« Reply #7 on: 31/10/2014 21:24:28 »
so the strongest pull of gravity on any other body is from the planet/star as a whole and pull of gravity on the surface of the planet/star weakens the deeper you go to the centre as there is zero gravity in the centre?

#### The Naked Scientists Forum

##### Re: a question of gravity
« Reply #7 on: 31/10/2014 21:24:28 »