# The Naked Scientists Forum

### Author Topic: How far does gravity of an object reach?  (Read 2807 times)

#### acecharly

• Sr. Member
• Posts: 171
##### How far does gravity of an object reach?
« on: 22/12/2013 15:58:56 »
Taking the gravitational effect of a body how far does it reach? If we say its effect is a quarter of its strength when the distance is doubled due to the inverse square law, does this not make its attraction infinite although exceptionally weak at large distances. Also do we know why gravity is the strength it is? Ie why is the gravitational force of a body the mass of the sun what it is why is it not twice as much for that amount of mass. Finally if we took all the mass in the universe does its gravitational effect have any relationship to the universes size?

Cheers Ace

#### Pmb

• Neilep Level Member
• Posts: 1838
• Physicist
##### Re: How far does gravity of an object reach?
« Reply #1 on: 22/12/2013 16:15:36 »
Quote from: acecharly
Taking the gravitational effect of a body how far does it reach?
The effect is infinite.

Quote from: acecharly
If we say its effect is a quarter of its strength when the distance is doubled due to the inverse square law, does this not make its attraction infinite although exceptionally weak at large distances.
I don’t understand this comment. Why would you say does this not make its attraction infinite? You already noticed that the further away you are the weaker the field becomes. Therefore at an infinite distance the effect goes to zero.

Quote from: acecharly
Also do we know why gravity is the strength it is?
No.

Quote from: acecharly
Ie why is the gravitational force of a body the mass of the sun what it is why is it not twice as much for that amount of mass.
Nobody knows. It had to be something so there’d have to be a person asking why it had to be that value, right? :)

Quote from: acecharly
Finally if we took all the mass in the universe does its gravitational effect have any relationship to the universes size?
That depends on what you mean by “all the mass in the universe.” There is more to mass as the source of gravity than what you have in mind.

#### acecharly

• Sr. Member
• Posts: 171
##### Re: How far does gravity of an object reach?
« Reply #2 on: 22/12/2013 16:34:07 »
Thanks for the reply PMB, when i said does this not make it infinite, it was merely an expected result of my initial question. So your saying that the reason for its strength is unknown thats interesting. My point about the relationship between all the mass in the universe and its size was that if we knew how much mass there was and that there was a relationship we would be able to work out its true size. Also if it was rite then gravity would change in stregnth for a given mass over time id have thought as the universe expanded.

#### acecharly

• Sr. Member
• Posts: 171
##### Re: How far does gravity of an object reach?
« Reply #3 on: 22/12/2013 16:41:47 »
Just another thing, as mass is turned into energy over time through stars nuclear fusion would this not also make space expand faster as the mass decreased?

#### Pmb

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• Posts: 1838
• Physicist
##### Re: How far does gravity of an object reach?
« Reply #4 on: 22/12/2013 17:20:41 »
Just another thing, as mass is turned into energy over time through stars nuclear fusion would this not also make space expand faster as the mass decreased?
No. Why should i?

#### acecharly

• Sr. Member
• Posts: 171
##### Re: How far does gravity of an object reach?
« Reply #5 on: 22/12/2013 17:33:38 »
Yes if its holding back the expansion of the universe.

#### Pmb

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• Posts: 1838
• Physicist
##### Re: How far does gravity of an object reach?
« Reply #6 on: 22/12/2013 17:36:38 »
Quote from: acecharly
My point about the relationship between all the mass in the universe and its size was that if we knew how much mass there was and that there was a relationship we would be able to work out its true size.
Not really. There is more to it than that. As I said That depends on what you mean by “all the mass in the universe.”   Some people don't understand that how much mass an object has depends on it's speed. So we need to know more than its rest mass, we need to know its energy because E = mc2 and thus the more we know about its energy the more we know about its mass. Also the pressure of matter contributes to gravitational effects. That's how Dark Energy works. It acts like negative pressure. So dark energy contrubutes to the size and shape of the universe.

#### Pmb

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• Posts: 1838
• Physicist
##### Re: How far does gravity of an object reach?
« Reply #7 on: 22/12/2013 17:48:06 »
Quote from: acecharly
Yes if its holding back the expansion of the universe.
No. First off mass is never turned into energy. That's a common misconception in relativity. The amount of mass and energy never changes.  All that's happening is that the form of matter has changed. Nothing more. The amount of mass never changes because mass is conserved in nuclear reactions.

To see an example please see
http://home.comcast.net/~peter.m.brown/sr/nuclear_fission.htm

That shows what happens when a uraniuma atom fissions/splits into the two nuclei thorium and helium. Some of the mass changes from mass in the form of rest mass to mass in the form of kinetic energy. The total mass is conserved as the calculation shows.

Mass(U nuclei at rest) = Mass(moving He nuclei) + Mass(moving T nuclei)
= Mass(stationary He nuclei) + Mass(stationary T nuclei) + Mass(kinetic energy of T and He nuclei)

#### evan_au

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##### Re: How far does gravity of an object reach?
« Reply #8 on: 23/12/2013 08:36:41 »
Quote
does this not make its attraction infinite although exceptionally weak at large distances
The attraction between two objects decreases as their distance increases.
However, as you consider spheres of increasing radius, there is more matter within this radius.
Which effect "wins" depends on the density of matter in the universe.

In the other direction, the attraction between two bodies increases the closer you bring the bodies together. If you manage to get sufficiently large masses close enough, the free-fall velocity exceeds the speed of light, and you have a black hole, from which nothing can escape.

Quote
if we took all the mass in the universe does its gravitational effect have any relationship to the universes size
In some 20th century theories of the universe, there was a critical density, above which the gravitation of the universe would slow, halt, and eventually reverse the expansion of the universe, resulting in a "big crunch".

If the density were lower than this critical value, the universe could expand forever.

The discoveries of Dark Matter and Dark Energy in recent decades have affected whether cosmologists think our universe is above or below this critical density.

#### The Naked Scientists Forum

##### Re: How far does gravity of an object reach?
« Reply #8 on: 23/12/2013 08:36:41 »