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truedelphi

(32,324 posts)
Fri Dec 14, 2012, 04:01 AM Dec 2012

Can somebody explain a few thngs about gravity -

I found myself bombarded with questions after looking at this science article that "The Straight Story" posted

http://www.democraticunderground.com/?com=post&forum=1002&pid=1975319

If you read what is in that OP, there is a sentence that says that after a lot of mass is moved around, there is more gravity in some places, and less in others. (or at least, that is how my brain interpreted the sentence.)

I am not sure I follow that. I though that gravity is uniform. I mean, it is, isn' t it?

lob1

(3,820 posts)
1. Until a day or two ago, I thought gravity was consistent, too.
Fri Dec 14, 2012, 04:23 AM
Dec 2012

Then I saw a map show gravity variances, but it was a much more localized map than the world view this one presents. This article (and the first one I saw) left me with more questions than I started with. Like, they don't say how much the variances are. Can you run faster on Ventura Blvd. than Sherman Way?

Odin2005

(53,521 posts)
6. Density differences lead to very tiny gravity "anomalies"
Fri Dec 14, 2012, 09:35 AM
Dec 2012

On the moon the basaltic rock that makes up the lunar "seas" is denser than the anorthritic rock that make up the lunar highlands, so the "seas" show up as high-gravity anomalies.

These anomalies are extremely small and you cannot feel them.

truedelphi

(32,324 posts)
8. You might enjoy this anecdote -
Fri Dec 14, 2012, 03:07 PM
Dec 2012

We were watching the SF Giants in the Baseball playoffs. And one team they were playing just couldn't get any hits.

The commentators were saying, (And I wish I was making this up, but I'm not): "Since the sun has now gone down, it is colder, and the drop in temperatures makes it harder to hit the ball, as the ball is less of a target than when it is in the heat."

I assume that when it is very very hot a ball might indeed be noticeably larger. But what made the announcers comments so pathetic is that the Giants weren't having a hitting problem at all!

 

jtuck004

(15,882 posts)
11. LOL.
Fri Dec 14, 2012, 03:19 PM
Dec 2012


We so need to change the culture so philosophy and logic and thinking are seen as important again, quit rewarding at least the worst of the brainless talk.

I wonder if the movies have got it wrong. The zombies aren't coming. They're here.



Festivito

(13,452 posts)
3. Gravity is between two masses. Weight is the amount of attraction.
Fri Dec 14, 2012, 06:25 AM
Dec 2012

For example: Your body and the earth. Neither our bodies or our planet is uniform - mass wise.
Denser parts of our planet will attract more than less dense parts because mass will be greater in the denser areas.

Drop a hammer and the heavier denser hammer head will go toward the earth first.

Water is most dense at about four degrees Celsius. That would happen nearer the poles.

The interior of our planet is still a mystery. There are probably some things going on inside that would lead to changes in density (even in composition) that result in slightly different pull on some mass near the earth's surface.

If the density were less uniform, the planet would not be round. But, we also know the planet is not perfectly round, so we could also surmise that gravity would not be perfectly the same everywhere.

 

DCKit

(18,541 posts)
4. Like why does the earth have more pull than the mostly empty space around it?
Fri Dec 14, 2012, 06:30 AM
Dec 2012

It's about mass and proximity.

oldbanjo

(690 posts)
5. Wouldn't the height above sea level affect gravity,
Fri Dec 14, 2012, 06:38 AM
Dec 2012

also the distance around the Earth at the equator is further than other locations due to the spin of the Earth.

jeff47

(26,549 posts)
7. Gravity isn't based on sea level
Fri Dec 14, 2012, 11:10 AM
Dec 2012

The strength of the Earth's gravity is dependent on the material below you. The denser the material, the more gravity.

Also, the Earth doesn't bulge at the equator. The Earth is slightly pear-shaped - the southern hemisphere is slightly larger than the northern.

truedelphi

(32,324 posts)
10. I think there were indications in the article that that is so
Fri Dec 14, 2012, 03:10 PM
Dec 2012

For instance, there is a discussion about how they had to adjust for the sea level of the oceans.

caraher

(6,256 posts)
12. Gravity depends on the distribution of the matter "creating" it
Fri Dec 14, 2012, 03:59 PM
Dec 2012

Were Earth spherically symmetric, with density that depends only on distance from the center, gravitational acceleration would depend only on distance from the center of Earth and would be the same everywhere on its surface. Now for many purposes, taking Earth to be spherical is a fine approximation, and the variations caused by being at different elevations, deviations from a spherical shape, regions where less or more dense materials are near the surface, etc. are small. So from a "freshman physics" perspective, you just treat "g" as a constant with the essentially the same value everywhere.

In addition to real variations in gravity, there are also apparent variations in gravity related to Earth's motion. Apparent surface gravity is different at the poles compared to the equator not only because you are nearer Earth's center at the poles (thanks to the non-spherical shape of the planet) but because the planet rotates, causing a centrifugal force (yes, that is the correct term here, as we're considering Earth's surface to be an accelerating reference frame).

struggle4progress

(116,792 posts)
13. OK, so here: Like, ya wakes up in the morning, and ya decides to go to the diner for some chow,
Fri Dec 14, 2012, 05:41 PM
Dec 2012

like mebbe a cuppa joe and some biscuits with that brown gravity stuff that is OK. But the biscuits are uneven with steep sides and lumps and dimples, so when the chef pours the brown gravity over the biscuits it runs and it puddles and it doesn't go evenly, cuz it falls off the lumps on the sides of the biscuits and it falls into the dimples in the biscuits and the grease helps it slide down the sides, so finally there is more of the brown gravity here and less there. But a good cold beer can help wash it down, wherever it is stickin

DavidDvorkin

(19,181 posts)
14. Gravity is not uniform because the Earth's mass is not uniformly distributed
Fri Dec 14, 2012, 10:06 PM
Dec 2012

It varies greatly in density from one place to another. The Earth doesn't consist of just one sort of stuff, which is the same everywhere.

The difference isn't noticeable to human senses, but instruments can detect it.

The same is true of other bodies, of course. Google "lunar mascons" for an interesting story from the days of the Apollo project. Or you could Google "perilune wiggle", which should get you a lot of the same URLs, but which is a niftier phrase.

truedelphi

(32,324 posts)
15. Somehow none of the science teachers I had ever mentioned any of this -
Tue Dec 18, 2012, 06:32 PM
Dec 2012

They did mention that different celestial bodies had different "gravities" affecting them. I knew people were affected by "less gravity" when they were walking around on the moon.

But i did not know that there are anomalies here on earth depending upon the density. Though had I really thought about it - I'd have realized it.

For instance, most humans are very buoyant once in the water. Especially salty bodies of water, like TheGreat Salt Lake.

DavidDvorkin

(19,181 posts)
16. It's still common to teach gravity as though all bodies were point sources
Tue Dec 18, 2012, 07:00 PM
Dec 2012

The reality is much more complex, but teaching about the details would probably obscure the basic Newtonian laws.

Of course, Newton's laws aren't accurate, either, but they're fine for most cases -- speeds that aren't close to light speed and masses that aren't enormous.

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