# I Is gravity a constant?

1. Apr 12, 2017

### Benplace

If the Earth were divided into 4 sections and separated far enough that they have no gravitational pull on each other, would the gravity on each section be 25% of the current gravity on Earth?

2. Apr 12, 2017

### Voyager I

Yes, considering Newton's law of universal gravitation. See https://en.wikipedia.org/wiki/Newton's_law_of_universal_gravitation.
Put 0.25*m2 instead of m2. m2 is mass of our Earth.
F is 0.25 times the original gravitational force.

3. Apr 12, 2017

### A.T.

The OP seems to ask about surface gravity. I don't think it will be equal to g/4 in general.

4. Apr 12, 2017

### Benplace

Would the amount of star dust excreted from a dying star equal the gravitational force of the original star then?

5. Apr 12, 2017

### Staff: Mentor

I'm sorry, but that question doesn't make a lot of sense. Can you try to be more clear about what you're asking? Also, note A.T.'s post above - the answer that Voyager1 provided may not have been correct, depending on what you're trying to ask.

6. Apr 12, 2017

### Benplace

Sorry, I'm not a physicist or a mathematician, I am just interested.
From what I understand, when a star dies, heavier elements are extracted from it. Does the total sum of those elements and the gravitational force they create equal the gravity the star originally created by warping space? From what I understand we all warp the fabric of space creating a gravitational force even though a small amount.

7. Apr 12, 2017

### Khashishi

Gravity is just a function of mass. Well, actually, since you posted this in the relativity forum, it's more accurate to say it depends on the stress energy. But for ordinary, nonrelativistic matter, the stress-energy is dominated by the mass-energy.

So, it doesn't matter what the star is made of; just how massive it is. You also warp space, but not very much because you aren't very massive. The distribution of mass in a star can affect the gravitational field some, but stars are fairly close to spherical so the shape is not very important.

8. Apr 12, 2017

### Khashishi

Newtonian gravity is linear. If you double the mass (while keeping the same dimensions), you have double the gravitational force on a test object.

General relativity is nonlinear. If you increase mass enough (while keeping the same dimensions), you eventually get a black hole.

9. Apr 12, 2017

### Benplace

What I am trying to figure out, is it possible that when a star dies the mass ejected from it could be greater than the mass of the original star?

10. Apr 12, 2017

### Khashishi

Nope.

11. Apr 12, 2017

### pervect

Staff Emeritus
No. If you used an external source of energy to disassemble a star - or a planet - that would require you to put energy into the system to perform the disassembly. It takes work to overcome the gravitational binding energy holding the star or planet together, so the total mass of the system would increases after dissassembly.

But the increase in energy of the system can be traced to the external source - it wouldn't happen that way if a star exploded.

To get into more details would require a fairly technical discussion of mass in General Relativity, which isn't a B-level topic.

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