# A Change in Earth's Gravity?

Discussion in 'Architecture & Engineering' started by Medicine*Woman, Dec 9, 2008.

1. ### originTrump is the best argument against a democracy.Valued Senior Member

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A change in density would not change the gravity of a body.

3. ### danshawenValued Senior Member

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???

Density = mass/volume
Fg = G(m1m2)/r1^2;
Fg = force due to gravity, G = universal gravitational constant, m1 = mass near surface, m2 = mass of planet, r1 = initial radius of planet

Assuming the density changed by means of changing the volume of a fixed mass, orbits far from the surface will not change very much, but shrinking the radius to accommodate a change in volume would alter surface gravity:

Fg/m1 = (G*m2) / r2^2; r2 = 0.5 r1; r2 = radius of shrunken planet

If the radius is shrunk to 1/2 r1 with the mass of the body held constant, the acceleration due to gravity increases by a factor of 4 at the surface.

Last edited: Jul 20, 2014

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comical

7. ### KittamaruNow nearly 40 pounds lighter.Staff Member

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Technically speaking, he is correct - gravity is a function of mass, not density. The effect gravity has on an object DOES change with distance, but that is not what Origin is debating. Specifically, a change in the density of an object, without altering its mass, would not change its gravitational effect.

8. ### krash661[MK6] transitioning scifi to realityValued Senior Member

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" The density, or more precisely, the volumetric mass density, of a substance is its mass per unit volume. The symbol most often used for density is ρ (the lower case Greek letter rho). Mathematically, density is defined as mass divided by volume: p= m/v "

" In physics, mass is a property of a physical body which determines the body's resistance to being accelerated by a force and the strength of its mutual gravitational attraction with other bodies....Thus, any stationary body having mass has an equivalent amount of energy, and all forms of energy resist acceleration by a force and have gravitational attraction. "

" Newton's law of universal gravitation postulates that the gravitational force of two bodies of mass is directly proportional to the product of their masses and inversely proportional to the square of the distance between them. "

gravitation-
2. force that attracts things: the mutual force of attraction between all particles or bodies that have mass

mass-
5. physics: physical quantity: the property of an object that is a measure of its inertia, the amount of matter it contains, and its influence in a gravitational field.

density-
2. physics: relative mass: a measure of a quantity such as mass or electric charge per unit volume

then there is,
density function

9. ### krash661[MK6] transitioning scifi to realityValued Senior Member

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" The density, or more precisely, the volumetric mass density, of a substance is its mass per unit volume. The symbol most often used for density is ρ (the lower case Greek letter rho). Mathematically, density is defined as mass divided by volume: p= m/v "

" In physics, mass is a property of a physical body which determines the body's resistance to being accelerated by a force and the strength of its mutual gravitational attraction with other bodies....Thus, any stationary body having mass has an equivalent amount of energy, and all forms of energy resist acceleration by a force and have gravitational attraction. "

" Newton's law of universal gravitation postulates that the gravitational force of two bodies of mass is directly proportional to the product of their masses and inversely proportional to the square of the distance between them. "

gravitation-
2. force that attracts things: the mutual force of attraction between all particles or bodies that have mass

mass-
5. physics: physical quantity: the property of an object that is a measure of its inertia, the amount of matter it contains, and its influence in a gravitational field.

density-
2. physics: relative mass: a measure of a quantity such as mass or electric charge per unit volume

then there is,
density function

10. ### krash661[MK6] transitioning scifi to realityValued Senior Member

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i'm so clueless as to why it posted twice.

11. ### KittamaruNow nearly 40 pounds lighter.Staff Member

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It happens sometimes - website or web server hiccup.

Curious with your post - is that a refutation of what I said, in support of what I said, or just additional information?

12. ### originTrump is the best argument against a democracy.Valued Senior Member

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What you said is correct. Changing the density will not change the gravity of an object but if you were to move closer to the center of mass, such as staying on the surface that is closer to the center of mass you would definitely have a higher acceleration due to gravity. Still the gravity of the mass would not change.

13. ### originTrump is the best argument against a democracy.Valued Senior Member

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I am curious too. He seems to think increasing the density of a mass increases the gravity (comical?) yet as far as I can tell he wrote nothing in support of that incorrect notion.:shrug:

14. ### krash661[MK6] transitioning scifi to realityValued Senior Member

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i'm shocked you don't get it.

15. ### zgmcRegistered Senior Member

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How much gravity would we experience on earth if it had the same mass but half the radius?

Last edited: Jul 26, 2014
16. ### sculptorValued Senior Member

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gravity is a constantly moving/changing beasty
we have the earth's gravity, modified by ice and mountain building, modified by the earth moon system, modified by the other planets, and modified by the sun, then modified by our position on our arm of the galaxy

here's another pix of the anomalies on earth:

17. ### zgmcRegistered Senior Member

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No one knows? well if anyone figures this out, I am also curious what the effect of gravity would be if you were the same distance from the center of the planet as we are currently.

18. ### Boris2Valued Senior Member

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so if the Sun suddenly became a black hole the orbit of the earth would change, would it?

;-)

19. ### Janus58Valued Senior Member

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With the same mass and half the radius, the Earth's surface gravity would be 4 times stronger. If you move out to a distance equal to that of its present radius, the gravity at that point would be equal to the Earth's present surface gravity.

20. ### zgmcRegistered Senior Member

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756
Based on that, it seems that the gravity of a body really isnt "changed" by becoming more dense. The surface of the body is just closer to the gravitational center which leads to an increase of gravity on the surface.

21. ### Janus58Valued Senior Member

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Correct.

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Someone already has mentioned a BH.
Let's Imagine that our Sun suddenly and magically became a BH.
That would mean squeezing the entire volume of the Sun, into about 5Kms diameter.
Gravity would not change, and all the Planets would go on orbiting as they presently do.
The only thing that really changes, is the topological shape of the gravity well by the Sun/BH, and the only objects that may be affected by it, are comets and Asteroids that stray within a certain distance of the Sun/BH, remembering that a BH is not an all purpose infinite vacuum cleaner.

23. ### krash661[MK6] transitioning scifi to realityValued Senior Member

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keep in mind, radius and density are two different elements.