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## Earth's Gravity

If the Earth was impacted by a massive object, would this have changed the Earth's gravity? During the age of dinosaurs could Earth's gravity have been less that it is today?
Last edited by kryton; 2007-Apr-12 at 01:57 AM.

2. It would have to be an incredibly massive object. During the time of the dinosaurs, the gravity was, for all practical purposes, identical to the current gravity.

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Originally Posted by cjl
It would have to be an incredibly massive object. During the time of the dinosaurs, the gravity was, for all practical purposes, identical to the current gravity.

A fair reasonable estimate can be obtained by computing the total mass of all micrometeorite collisions with the Earth each day, on average (a fairly well-known quantity), and comparing it to the mass of the Earth's atmosphere we lose to outerspace each day (also a fairly well-known quantity).

The earlier years of our planet's existance saw both higher rates of impact from larger particles, as well as a higher pressure atmosphere with greater rate losses than today. Since I know neither, I'm not qualified to comment as to the net effect over time.

I would hazard a guess that it was not materially different than it is today, and possibly the Earth is more massive today, as the Earth's atmosphere is an exceedingly small component of it's overall mass.

4. Originally Posted by kryton
If the Earth was impacted by a massive object, would this have changed the Earth's gravity?
Several billion years ago, the Earth's net mass probably was increased with a roughly Mars mass object that struck the Earth, resulting in the formation of the moon. Of course, life could not survive such an impact.

During the age of dinosours could Earth's gravity have been less that it is today?
No. The Earth's mass has not changed significantly for billions of years. If there had been a massive impact so recently, this would be a dead world. Even the mass of a several mile diameter asteroid, like the one thought to have wiped out the dinosaurs, is irrelevant compared to the mass of the Earth.
Last edited by Van Rijn; 2007-Apr-11 at 05:38 AM. Reason: typo

5. Originally Posted by Van Rijn
Several billion years ago, the Earth's net mass probably was increased with a roughly Mars mass object that struck the Earth, resulting in the formation of the moon. Of course, life could not survive such an impact.
I was going to object to this, but I think it depends on what one means by "several". I always thought "several" implied two, or at the most three. But maybe it can be more. Apparently the impact was something like 4.5 billion years ago, and the earliest recorded life is from about 3.5 billion. It seems likely that life didn't even exist on earth at the time of the impact.

6. Originally Posted by Jens
I was going to object to this, but I think it depends on what one means by "several". I always thought "several" implied two, or at the most three.
When I was a child, I was told, "You've been naughty several times today." I asked what several meant, and was told, "Any number except one and two."

I don't think that's quite right - I don't think fifty counts as several - but I think the word means "a fair few".

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Multiple duplicate posts

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Dup. I'll never post at back up time again. Ugh.

9. Originally Posted by Jens
I was going to object to this, but I think it depends on what one means by "several". I always thought "several" implied two, or at the most three. But maybe it can be more. Apparently the impact was something like 4.5 billion years ago, and the earliest recorded life is from about 3.5 billion. It seems likely that life didn't even exist on earth at the time of the impact.
From http://dictionary.reference.com/browse/several

1. being more than two but fewer than many in number or kind: several ways of doing it.

I could have been more specific, but I was writing quickly. Also, I did not state that there was life on the Earth at that time, but did note that life could not survive such an impact. That's one of the reasons we know there haven't been any more recent impacts on this scale. Objects with relatively insignificant mass can cause mass extinctions. After something like this, though, observing aliens would have to rewrite their equivalent of textbooks about the planet.

10. Innumerable might be a better choice; I certainly wouldn't like too try to count the particles that resulted from the Big Splash, as it is sometimes called.
An image of the impact by Fahad Sulehria
The first few minutes after impact
Look at all those pretty particles.

11. Originally Posted by Jens
Apparently the impact was something like 4.5 billion years ago, and the earliest recorded life is from about 3.5 billion. It seems likely that life didn't even exist on earth at the time of the impact.
Right. Astrobiology Magazine clarifies....

Excerpt: Does the first evidence of life date to 3.85 billion years ago (Ga), or 3.65 Ga? A 200-million-years discrepancy may seem trivial almost 4 billion years after the fact. And yet scientists continue to debate whether some of the oldest rocks ever found date to 3.85 Ga, or "just" 3.65 Ga.

The discrepancy matters because the rocks, however old they are, indicate that life already existed at the time they formed. The dispute is not just a matter of how early life began, however, but under what conditions: The earlier date was during the tail end of an asteroid storm called the "late heavy bombardment," while the later date was after the bombardment ceased.

12. I, personally, use "several" to mean anywhere between two and a dozen. (Okay, sometimes, I use it to mean more than that, but I always catch myself and promptly feel dumb.)

I can't help much with gravity, but definitions? Oh, yeah.

13. Several billion years ago, the Earth's net mass probably was increased with a roughly Mars mass object that struck the Earth, resulting in the formation of the moon. Of course, life could not survive such an impact.
If a really large object(close to Earth's size) with a great mass(equal to, or greater than Earth) does hit Earth, would it be possible for earth to just "break" into half or whatever number of pieces?

14. Originally Posted by unknownspiritx
If a really large object(close to Earth's size) with a great mass(equal to, or greater than Earth) does hit Earth, would it be possible for earth to just "break" into half or whatever number of pieces?
It would need to overcome Earth's self-gravity, but if it's massive enough and fast enough, it's possible the Earth could be disrupted. The planet wouldn't literally crack in half, of course.

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Any impact that could break the Earth into separate large pieces
would scatter vast numbers of smaller pieces all around. Everything
would be enveloped in a cloud of hot vapor comparable in mass to
the total mass of the pieces. It would cool rapidly, with some vapor
condensing onto the solid pieces, but some of it would be blown
away by solar wind. The impactor would have to be moving very
fast to knock any material into an orbit far from Earth's orbit.

The total increase in mass of the Earth from infalling meteoroids
and asteroid impacts over the last two billion years is miniscule
compared to the mass of the Earth. Earth is pretty big.

-- Jeff, in Minneapolis

16. Several., seems to contain the word seven. So more than a few wouldn't you think...
No. planet Earth can not sustain its form as a half or any other recognizable part of the original sphere. The force of impact would completely destroy this planet. Some million years or so later the newly coalesce material would have settled into a spherical shape. Gravity would do this. But life would not return until the conditions became tolerant of it. This could take hundreds of millions of years. Or never.

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"Several" and "a few" are synonyms, but used differently:

I only took a few cookies.
We only have a few miles to go.
I got A's in several classes.
You fouled up in several ways.

I use "a couple" to mean "about two or three or so".

I caught a couple fish.
Life has existed on Earth for a couple billion years.

eburacum, did you misunderstand what the "several" applied to?

-- Jeff, in Minneapolis

18. Oh yes; I thought it was referring to the bits flying off the Earth in the impact. mea culpa.

I myself would use several to refer to at least three, and perhaps as many as twenty.

19. Ah, so its all upto gravity to make it turn back into a sphere shaped object again? But I don't get the whole idea of gravity. Where does it even come from? I understand where gravitiy might come from from a planet or star (is it because of the core of the planet/star ?) But i'm a little confused about where the gravity comes from in just a vaccum area?

20. If it has mass it has gravity. The more mass the more gravity. Density is not important. The sum total of mass of a galaxy exerts its gravity as a whole.
Yes if a number of particles are large enough gravity will pull it into a sphere. Density will increase as mass increases.

21. Originally Posted by astromark
If it has mass it has gravity. The more mass the more gravity. Density is not important. The sum total of mass of a galaxy exerts its gravity as a whole.
Yes if a number of particles are large enough gravity will pull it into a sphere. Density will increase as mass increases.

so space itself has no gravity? (or would have no gravity if there were no objects with mass in it?) its just all the objects that are in space that creates gravity between each other?

Oh and one more question, is it always true that density will increase as mass increases?

which means increase in mass= increase in gravity=increase in density

Anyways, your explanation was simple but fantastic. I think i understand the mass-gravity relation well now. Thanks

22. Originally Posted by unknownspiritx
so space itself has no gravity? (or would have no gravity if there were no objects with mass in it?) its just all the objects that are in space that creates gravity between each other?

Oh and one more question, is it always true that density will increase as mass increases?

which means increase in mass= increase in gravity=increase in density

Anyways, your explanation was simple but fantastic. I think i understand the mass-gravity relation well now. Thanks

unknowspiritx,
You can increase the density of an object and decrease the mass of the object at the same time. Space is full of gravity, mass doesn't contain gravity. Mass displaces the space around it, this dispacement is gravity. Gravity is a push to a lower force from a higher force. The energy around an object expandes faster than the masses energy, because mass is a condensed form of energy, these are the high and low forces that give us gravity. You're not going to find that in any physics books back in class, but it doesn't mean that it can't be true. Actually using this thought prosses answers questions that can't be answered by scientists right now.

23. Originally Posted by rebel
You're not going to find that in any physics books back in class, but it doesn't mean that it can't be true. Actually using this thought prosses answers questions that can't be answered by scientists right now.
This sounds like a good item for the ATM forum. Present your idea, with supporting evidence, there and we can discuss it and ask questions about it. It is not, however, appropriate for the Q&A forum.

24. Originally Posted by Van Rijn
This sounds like a good item for the ATM forum. Present your idea, with supporting evidence, there and we can discuss it and ask questions about it. It is not, however, appropriate for the Q&A forum.
Van Rijn,
If you have a question don't be afraid to ask.

I also have an invisible elf trap 100% gauranteed to eliminate your elf problem in your backyard.

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## Jack-in-the-Box energy has gravity, too

Originally Posted by astromark
If it has mass it has gravity. The more mass the more gravity. Density is not important. The sum total of mass of a galaxy exerts its gravity as a whole.
Yes if a number of particles are large enough gravity will pull it into a sphere. Density will increase as mass increases.
astromark. Energy has gravity, too. E=mc2. So, m=E/c2
So if you have energy in the form of photons or neutrinos...that too has an equivalent mass. Suppose you have in your hand a box with some gas inside, and a few slightly greater than ~1.022 Mev photons....just enough to create electron/positron pairs. We magically give the box perfect internal wall mirrors, as suggested in another thread elsewhere, and add the additional condition that the atoms of the internal gas can facilitate the creation of the particle/antiparticle pairs, but not absorb any of the photon/neutrino energy as they do so. (We get to be control freaks here..but this is the sort of stuff theorists do).
Now, we set the gears in motion. The traveling neutrinos/photons, chugging along at c, hit an atom and create a pair of particles with rest mass each of 511 kev, or 0.511 Mev....the electron, and the positron. Photon/neutrino energy of E=hv has "disappeared", simultaneously two particles with non-zero rest mass have"appeared". Is the box "heavier"? No.
The two non-zero rest mass particles travel a bit and annihilate, back into the 1.022 photon/neutrino. Is the box lighter? No. The total mass/energy is constant. So the total gravitational field is constant. The strength of the gravitational force felt by you and the box in your hand is still Fg= G (mbox)(massyou) / (separation of centers of mass)2......we'll put you in space to eliminate Earth here.
To think otherwise, whenever the pair forms, Fg goes up....and when they annihilate, Fg goes down. Now if we made our conditions more exotic with a much more energetic photon, or neutrino say 10Google ev....you'd have a wildly oscillating Fg. not going to happen. (Epstein, Thinking Physics...a great little book). As you sit with your hot meal...a hard boiled 212 F egg...it has a little more field than it does when it cools off by radiation, conduction and convection...no magic mirrors there.
I need also to stipulate that the neutrinos in the box never mirror, when they form, they pair form as electron/positron and then annihilate back to photon before reaching the edge of the box....otherwise I'd be suggesting lepton number conservation failure, something never seen in a particle physicist's world. pete.
Last edited by trinitree88; 2007-Apr-16 at 10:16 PM. Reason: subscript boxed wrong..my fatigue is showing, apostophe

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For the second thing you need to know about gravity, do a web
search for "Cavendish experiment".

Different kinds of rock have different density, but typical is about
three times the density of water, or a density of three. A rock with
a density of three and a volume of one cubic meter has a mass of
three thousand kilograms. A BIG rock with a density of three and a
volume of one cubic kilometer has a mass of three trillion kilograms.
A REALLY big rock of the same kind, comparable to the size of the
Moon, with a volume of one trillion cubic kilometers, would have
enough gravity to compress the rock in the interior slightly, so
that it might have an overall density of 3.2, and its mass would
be 3.2 x 10^24 kg.

A larger body such as the planet Mercury or the Earth will be more
compressed, and so will be even denser. Because Earth has a large
iron core, and iron has a density of about seven, and the core and
all the rest of the rock in the Earth is significantly compressed by
the tremendous weight, Earth's overall density is about 5.5, the
most dense of all the planets of the Solar System. Mercury is the
next most dense at 5.4.

But I'm not sure I understood your question.

-- Jeff, in Minneapolis

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What if the Earth was impacted by a dead star? Neutron or pulsar etc. would this affect the gravity. Is this even realistic?

28. Originally Posted by kryton
What if the Earth was impacted by a dead star? Neutron or pulsar etc. would this affect the gravity. Is this even realistic?
The earth would add a very thin layer to the neutron star. The mass of the earth is tiny compared to that of a neutron star, which would have roughly half a million to a million times as much mass. So the neutron star's gravity would be very slightly affected.

But, while it is theoretically possible a neutron star could come rolling through the solar system, it is very, very unlikely.

29. Originally Posted by Van Rijn
The earth would add a very thin layer to the neutron star. The mass of the earth is tiny compared to that of a neutron star, which would have roughly half a million to a million times as much mass. So the neutron star's gravity would be very slightly affected.

But, while it is theoretically possible a neutron star could come rolling through the solar system, it is very, very unlikely.
Would a neutron star be of any danger to planet Earth IF it did approach our solar system?

30. Originally Posted by unknownspiritx
Would a neutron star be of any danger to planet Earth IF it did approach our solar system?
If it got close enough, definitely.

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