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Author Topic: Why do black holes have more gravity than anything else?  (Read 505 times)

Offline thedoc

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Byron Donovan asked the Naked Scientists:
   Why do black holes have more gravity than anything else?
Doesn't a black hole have the same mass as the star that it was created from and doesn't an object's gravitational field depend on its mass? Or is it that if that mass takes up less space it becomes more gravitational? In other words does a pound of gold actually weigh more than a pound of feathers?
What do you think?
« Last Edit: 22/08/2016 16:53:01 by _system »


 

Offline syhprum

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It is a matter of size the strength of gravity depends not only on the mass of the objects but upon the distance between them.
there are a lot of unknowns about black holes with talk of singularity's so let us consider their close relatives neutron stars in these although the mass may be only about twice that of the sun the matter is so compressed that the radius is about 10Km whereas a normal star of the same mass would have a radius of 500,000Km therefore a body on the surface would be subjected to gravitational force 50,000 times as great due to its proximity to the centre of gravity.
 

Offline PmbPhy

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Byron Donovan asked the Naked Scientists:
   Why do black holes have more gravity than anything else?
Doesn't a black hole have the same mass as the star that it was created from and doesn't an object's gravitational field depend on its mass? Or is it that if that mass takes up less space it becomes more gravitational? In other words does a pound of gold actually weigh more than a pound of feathers?
What do you think?
Black holes don't have more gravity than anything else. They're merely so compact that you can get closer to them and since the strength of the field increases as you get closer they appear to have more gravity, but they don't. Recall that the strength of a gravitational field is g = GM/r2. This only holds outside of spherical bodies or so far from an object that the shape of the object is negligible. Thus for a star this only holds outside the star and the strength of the field decreases linearly when inside. But a black hole can be merely inches in diameter so by the time you get that close the tidal forces would have shredded you by the time you got there.
 

Offline evan_au

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The above responses have talked about the strength of the gravitational force close to a neutron star or black hole.
A related way of looking at this question is the escape velocity, which is a measure of the energy required to launch an object from the surface.

If a neutron star has 1/50,000 the radius of the original star (but the same mass), then the escape velocity will be over 200 times greater than the original star.

A black hole is crushed down to such a small size that the escape velocity exceeds the speed of light. And, according to Einstein, you can't accelerate any massive object to a speed faster than light, so nothing can escape - not even light.

See: https://en.wikipedia.org/wiki/Escape_velocity
 

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