Where is Earth's gravity stongest?

reply to post by nobody you know


Those anomalies are only applicable to people who say things like "milligals" and "astrobleme". They represent hundredths of a percent deviation from standard gravity.

Consider a hyperthetical planet, the same size as Earth, with the following attributes:

It has a large iron inner core.
The outer core, mantle and lithospere have the mass/density of polystyrene.

Would you not agree that gravity would be strongest on the surface of the iron core rather than on the polystyrene shell?

EDIT - In fact it's the same argument as whether gravity is strongest on top of, or inside, the atmosphere.

reply to post by BagBing


Nope. The polystyrene and the iron inner core are a single mass. Gravity is still strongest at the surface.

Originally posted by CLPrime
reply to post by BagBing

 

Nope. The polystyrene and the iron inner core are a single mass. Gravity is still strongest at the surface.

I edited the above after you posted..

I said: In fact it's the same argument as whether gravity is strongest on top of, or inside, the atmosphere. Gravity should be greatest on top of the atmosphere...

reply to post by Arbitrageur


Actually a friend of mine, who was the Master/Captian (retired now) of a large research vessel that performed SRO operations in the Indian Ocean, told me that there is an area between India and Mauritius that actually produces a valley in the ocean because of a gravitic anomaly. The valley spans around 200 miles and has a total drop of about 100 feet. He said that the area is generally avoided because the anomaly forces ships to use more fuel. He could not avoid it though because of research and SRO operations. I have his maps showing the area somewhere on my computer but I would have to find them as they are related to another event involving a search and recovery operation.

So, I expect gravity is at a high point between India and Mauritius, since it is lowering ocean levels in a specific area.

Cheers - Dave

reply to post by BagBing


The atmosphere has no surface. It is diffuse layer of accumulated gas, and it has very little gravitational pull. You certainly can't be comparing the atmosphere and the Earth with the lithosphere/mantle/outer core and the inner core. I believe that's the very definition of reductio ad absurdum. Even the difference in mass between the polystyrene and iron core isn't that great.

The deviation of the Earth from a perfect sphere is hardly enough to create any significant gravitational deviation capable of locating any maximum below ground.

reply to post by bobs_uruncle


There is no scientifically observed anomaly of that sort. If there is such a valley, then it would cause a slightly lower than normal gravitational pull due to the lack of contributing mass. In fact, there is such a slight deviation in the area, around Sri Lanka. But, it's not nearly enough to cause any noticeable difference in fuel consumption. It's not even noticeable if your standing there on a really accurate scale.

Originally posted by BagBing
I said: In fact it's the same argument as whether gravity is strongest on top of, or inside, the atmosphere. Gravity should be greatest on top of the atmosphere..

Yes, this is what I was thinking when I responded earlier, that you'd need a large density gradient to have gravity increase as you descend, and the atmosphere versus the Earth provides a sufficiently large density gradient.

However to get a large enough density gradient below the Earth's surface would take something truly unusual, like maybe a huge deposit of solid lead, that's just below a large cave system which is mostly hollow, which in effect is still using the atmosphere to create the large gradient since the cave spaces would be filled with air. In most cases though, you probably won't get an increase below the surface unless you have something like an air-filled cave above you and something exceptionally dense below you.

Here's a good paper about the Earth's gravity if you're interested in this; it's not very technical, it's got a little high school math though:

Introduction to Earth Gravity

Scroll down the the very end of the pdf to the map of Earth's gravity, and you might be able to see the gravity anomaly the ship captain was talking about.

Originally posted by CLPrime
reply to post by bobs_uruncle

 

There is no scientifically observed anomaly of that sort. If there is such a valley, then it would cause a slightly lower than normal gravitational pull due to the lack of contributing mass. In fact, there is such a slight deviation in the area, around Sri Lanka. But, it's not nearly enough to cause any noticeable difference in fuel consumption. It's not even noticeable if your standing there on a really accurate scale.

Actually I think it does show up on the map at the end of the "Introduction to Earth Gravity" pdf I posted, and it's about where he said it was, a little closer to India, and it's about the right size...it's the only purplish area on the whole map.

However I'd have to agree that the claim of a related fuel consumption increase would require some evidence to substantiate the claim.

reply to post by Arbitrageur


Thanks for all the replies... it was an interesting question to begin with, for which I still don't have a definative answer!!!

I'm fairy certain my assumption that the highest gravitational pull would be under the surface is correct.

If I'm standing at the south pole, I would feel less gravity than someone next to me in a six foot ditch. The difference would be irrelevant of course, but would still exist.

Originally posted by BagBing
If I'm standing at the south pole, I would feel less gravity than someone next to me in a six foot ditch. The difference would be irrelevant of course, but would still exists.

So you don't believe my claim that you'd be essentially "weightless" at the center of the Earth, if you could get there somehow?

reply to post by Arbitrageur


You'll notice, I mentioned that. My assumption was that, since it's claimed to increase fuel consumption, then it must be thought to be an increase in gravity (I'm not sure why the average person would assume weaker gravity would increase fuel consumption). This is not the case because, as we both said, there is an ever so slight negative deviation from standard gravity in the area, which would certainly not be noticed in any way from a ship travelling in the area.

Originally posted by Arbitrageur

Originally posted by BagBing
If I'm standing at the south pole, I would feel less gravity than someone next to me in a six foot ditch. The difference would be irrelevant of course, but would still exists.

So you don't believe my claim that you'd be essentially "weightless" at the center of the Earth, if you could get there somehow?

Not at all - I stated before that there's zero g at the centre, and highest g at the poles. I think the g measurement as you go down though is very complicated because of the layered nature of the planet.

reply to post by BagBing


The confusion seems to be with the definition of "surface of the Earth." The surface of the Earth is not the ground, per se. Rather, the surface of the Earth is, basically, the averaged radial distance from the center to the ground. This distance defines a perfect sphere, and the location of the ground doesn't matter. At this single radial distance, the Earth's gravity is at a maximum.

Now, that doesn't mean that the ground can't dip above or below this spherical surface. But, we can't be concerned with that, because taking such variation into account is too tedious. You could say that the maximum gravity is below the ground, at the ground, and above the ground, and, for certain places on the planet, each of those would be right. However, Earth's gravity maximum is at the surface. Always. It doesn't matter what the density of each layer is, so long as each layer is roughly symmetrical.

reply to post by CLPrime


In working from the notion of a perfectly smooth, layered, oblate spheroid.

Don't get me wrong, my initial answer to the question "where is gravity strongest" was the south pole (the north pole was too problematic being an ice covered ocean).

But the more I think about it the less convinced I am. That's why I mentioned the atmosphere - if you could variably adjust it's density, you effectively change the point of peak gravity (assuming you are standing on a platform, rather than freefalling).

I'll to make some graphics tomorrow which I think better illustrates my point.

Originally posted by BagBing
reply to post by CLPrime

 

In working from the notion of a perfectly smooth, layered, oblate spheroid.

It doesn't matter what shape the Earth is, inside or out. The gravitational surface of the Earth is still a perfect sphere. This would be the case even if the Earth were a cube.

I cannot tell you that....you humans would want to either try to make money off it or weaponize it

Originally posted by CLPrime
reply to post by BagBing

 

The confusion seems to be with the definition of "surface of the Earth." The surface of the Earth is not the ground, per se. Rather, the surface of the Earth is, basically, the averaged radial distance from the center to the ground. This distance defines a perfect sphere, and the location of the ground doesn't matter. At this single radial distance, the Earth's gravity is at a maximum.

Now, that doesn't mean that the ground can't dip above or below this spherical surface. But, we can't be concerned with that, because taking such variation into account is too tedious. You could say that the maximum gravity is below the ground, at the ground, and above the ground, and, for certain places on the planet, each of those would be right. However, Earth's gravity maximum is at the surface. Always. It doesn't matter what the density of each layer is, so long as each layer is roughly symmetrical.

I don't this is correct. If the mass density in the core is higher than on the surface, the maximal gravity would be below the surface. Could well be the case for earth.

Edit: wiki says that the core is indeed more dense (as one would expect) so my answer would be below the surface (but not far). And it would be at the poles when you consider the oblate spheroid shape.

reply to post by -PLB-


And what about all the mass that's suddenly found itself outside this maximal point? Doesn't it also contribute to the total gravitational field of the planet?
As long as the internal structure is symmetrical, the maximal gravitational potential will be the gravitational surface. As I noted, however, this gravitational surface is not always located at ground-level, but the radial distance of the ground is too chaotic to account for. We might as well say the maximal potential could very well be located anywhere and everywhere relative to the ground.

reply to post by CLPrime


What I mean is that on average (imagine the earth a perfect sphere) its below the surface.