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ritwik
2012-Sep-19, 07:35 AM
gravity of BH is directly proportional to mass of BH

gravity of Earth is directly proportional to mass of earth

so why is density of BH & earth supposed to be differend ?

density increases with gravity and to increase gravity more mass is required so the proportion stays same !

so why is earth's garvity not scrunching mass of earth like that of a BH ?

gravity : mass of earth = gravity :mass of BH

Tog
2012-Sep-19, 08:38 AM
The gravity you feel depends on your distance from the center of the mass. If Earth were replaced by an Earth-mass black hole, the moon would never feel the difference. Nothing would change.

Here at the surface, we feel gravity as being a value of 1. As long as we had something to support us, we wouldn't notice any change in gravity either.

If that gravity were the result of an equal mass the size of a golf ball, we could cut our distance to the center if it in half and feel 4 times the gravity. Every time we cut the distance in half, gravity goes up 4 times from the previous step, all the way to the surface.

This is how spaghttification happens. Things eventually reach a point where the distance from the center to the close edge (your feet) and the distance from the center to the opposite edge (your head), is enough to pull each part with a much different level of force. Your head might be experiencing 64 g while your feet are getting 2048.

Since the Earth is less dense, the surface is as close as we can get to the center without putting some mass "above" you. This mass is less, but far closer. If you were to hollow out the earth and stand on the inside of the crust, the gravity pulling you "down" (toward the bit you're standing on), will be offset by the greater, though much further away, mass on the opposite side. The net result will be zero gravity on the inside of the hollow sphere.

When you step to the "outside" of the sphere, all the mass is now below you and pulls you toward the center.

Tog
2012-Sep-19, 08:40 AM
Or, more simply, a kg of gold and a kg of feathers have the same mass, and therefore the same gravity. The density is not the same.

Shaula
2012-Sep-19, 03:49 PM
so why is earth's garvity not scrunching mass of earth like that of a BH ?
Because the materials the Earth is made of resists compression. That is why you have a series of hard limits to the types of matter you find out there. If we assume no radiation pressure, just a lump of stuff, you get something like:
M < 1.4 sol = Normal matter, held up by electromagnetic forces or electron degeneracy pressure
1.4 sol < M < 5 sol = Neutron star, only neutron degeneracy pressure holds them up
M > 5 sol = Black hole

ritwik
2012-Sep-21, 01:10 PM
so if earth were a sphere of light elements of same mass as of now ...earth would have shrunken more !!

thanks..

korjik
2012-Sep-21, 01:25 PM
so if earth were a sphere of light elements of same mass as of now ...earth would have shrunken more !!

thanks..

No, it would depend on the density, not the material. In general, a lighter element has a lower density, but that isnt always true. Even then, the lower density would make the Earth larger, not smaller.

ritwik
2012-Sep-21, 01:50 PM
No, it would depend on the density, not the material. In general, a lighter element has a lower density, but that isnt always true. Even then, the lower density would make the Earth larger, not smaller.

garvity may not coalesce light elements as fast as hard elements but i guess if given time earth would become smaller ALWAYS

Hornblower
2012-Sep-21, 02:04 PM
garvity may not coalesce light elements as fast as hard elements but i guess if given time earth would become smaller ALWAYS

You guessed wrong. For something on the order of Earth's mass, a cold ball of ice will always be larger in diameter than a cold ball of iron of the same mass. The self-gravitation is nowhere near strong enough to crush the atoms.

Guessing does not do very well in physics. Studying the fundamentals will give much better results.

noncryptic
2012-Sep-25, 02:59 PM
Or, more simply, a kg of gold and a kg of feathers have the same mass, and therefore the same gravity.

more complete:

Same gravity at the same distance from the center of gravity of either object.
But the minimum distance to the center of gravity of a kg of gold is smaller than the minimum distance to the center of gravity of a kg of low density material such as feathers, so gravity at the surface of a kg of gold is larger than at the surface of a kg of feathers.