# Why we, or maybe I only, that the Earth is spherical in shape.

Discussion in 'Physics' started by Lightfire, Jan 14, 2013.

1. ### Lightfire Thread Starter Well-Known Member

Oct 5, 2010
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Okay, look at road, it is straight right? I really have no doubt that Earth is truly oblete spheroid or whatever.

But I'm just wondering why can't we feel. it. But yes, when I look videos taken above from helicopter, I can see the bulge of Earth.

Also, I always read "gravity is the attraction between two objects". In Earth, what's that two objects Maybe my pen and earth?

Thanks so much!

2. ### takao21203 AAC Fanatic!

Apr 28, 2012
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Because of the dimension human/earth it is almost the same as a continuously flat surface.

Physics where the grav. force comes from a point is wrong of course. It is only a model. In reality is it equivalent, but indeed, grav. force comes from each individual atom. You could not really calculate that properly.

So a model is used where the force comes from the centre of the sphere. It is actually wrong, and hard to understand, but the only way to pack it into a simple formula.

3. ### Markd77 Senior Member

Sep 7, 2009
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If you add up all the forces from every bit of the earth (force has direction) you find the force on the pen is towards the centre of the earth. It's a complex bit of maths that you will probably get to in a while.
The pen is probably a good example. If you balance it on your finger you find that for the purposes of gravity it seems like there is a single point that is attracted to the ground. Of course it is actually every single part of the pen.
Gravity is really weak, so the force between two pens is so small it would be very hard to measure, but it has been done for fairly small objects:
http://www.leydenscience.org/physics/gravitation/cavend.htm
To actually feel any force, at least one of the objects has to be really massive, like the earth.
Even something like a mountain makes very little difference to the local gravity compared to the rest of the earth.

4. ### Lightfire Thread Starter Well-Known Member

Oct 5, 2010
690
21
Why it is almost as a continuously flat surface?

**
EDIT: For example the force of attraction between two object is 1040.361*10−11 N. Then where's the direction of the force? (I always think of this as like magnet.)

Last edited: Jan 16, 2013
5. ### Markd77 Senior Member

Sep 7, 2009
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The same magnitude of force will be on both the boy and the girl and the directions will be directly towards the other's centre of gravity.
It's a very small force, as you have found, so it's not usually a good idea to increase your mass to make girls more attracted to you.

6. ### Lightfire Thread Starter Well-Known Member

Oct 5, 2010
690
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You mean the direction of boy will be to girl and vice versa because of the force?

Hahaha, bad idea actually I found that example somwhere in the internet!

7. ### takao21203 AAC Fanatic!

Apr 28, 2012
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Gravitation is not much of a deal here.
http://mary-mcgregor.deviantart.com/art/BB-Takao-n-hiromi-oekaki-LSNSC-291725088

Even if there is a manga with that title (which I do not have here for reference and also don't read online).

So you want to discuss

a)manga
b)human relationship
c)physics

?

For the physics it is not relevant if the objects are animate or not.

If you want to talk about manga AND physics, the Akira cartoon animation turns pretty weird towards the end.

More there is even one scene where a girl explains the making of the world to a boy.

But, actually Ameba's don't build houses

So if you watch Akira, you can get boy/girl talking to each other, physics, and it's a cartoon animation as well. a), b), and c).

8. ### thatoneguy AAC Fanatic!

Feb 19, 2009
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Having a lot of money will change this attraction formula greatly.

9. ### Metalmann Active Member

Dec 8, 2012
692
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Old, bald guys, with bunches of cash; make the attraction thing work well.

10. ### thatoneguy AAC Fanatic!

Feb 19, 2009
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Oddly enough, increased mass tends to be inversely proportional to attraction, sort of a "negative gravity".

11. ### takao21203 AAC Fanatic!

Apr 28, 2012
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491
Spiders can hang upside down in their webs all the time. They can climb up walls. They can hover up and down a silk rope.

12. ### Markd77 Senior Member

Sep 7, 2009
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595
Yes, there are always forces on both objects (Newton's third law [http://en.wikipedia.org/wiki/Newton's_first_law#Newton.27s_third_law]), the earth is also attracted to you with the same magnitude of force as you are to it.
Same with the moon and the earth, some nice animations here:
http://en.wikipedia.org/wiki/Earth-Moon_barycenter

13. ### Lightfire Thread Starter Well-Known Member

Oct 5, 2010
690
21
Actually, I do not intend to dicusss anime or so. I just made boy and a girl as an example because I saw it in the internet when I look up for how to claculate gravitation

Reference: http://www.school-for-champions.com/science/gravitation_force_objects.htm

You will see Force Atracting Boy and Girl...

I am not fan of animes...
I should have made another example...

14. ### takao21203 AAC Fanatic!

Apr 28, 2012
3,682
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I don't mind it. Means it does not upset me.
But it is not scientific question. If it is a boy or girl or a bag with concrete dust is irrelevant for the physics matter of gravitation.

And yes no anime so we don't need to mention it anymore.

In the west we are largely obsessed to keep education politically correct.

It's like if you discuss viscosity or boiling point and mention green lemonade. The color is not relevant at all.

You could of course say green lemonade Funai brand but that does not add useful information at all.

As I say, I don't really have a worry about it or a problem, use descriptions as you see fit.

15. ### Lightfire Thread Starter Well-Known Member

Oct 5, 2010
690
21
Sorry for misunderstanding. What is politically correct?

So should I mention object 1 with mass of x kg and object 2 with mass of y kg instead of using very generic names?.

I apologize if sometimes I ask like the way I ask earlier. Truly, East and West have many differences so I say sorry if sometimes our tradition does not jive well. Oh, I represent my country's education. No, no.
***Base from Wikipedia
Political correctness (adjectivally, politically correct; both forms commonly abbreviated to PC) is a term which denotes language, ideas, policies, and behavior seen as seeking to minimize social and institutional offense in occupational, gender, racial, cultural, sexual orientation, certain other religions, beliefs or ideologies, disability, and age-related contexts, and, as purported by the term, doing so to an excessive extent.

It's not my intention to offend any gender here!

**********

I have removed the boy and girl in my earlier post....

Last edited: Jan 16, 2013
16. ### WBahn Moderator

Mar 31, 2012
24,061
7,466
I heard of a sci-fi story in which a new civilization had only two laws:

1) It shall be unlawful to offend anyone.

The thinking here is that if you rob someone, that would probably offend them.

2) It shall be unlawful to be too easily offended.

We definitely need a dose of that one!

17. ### Metalmann Active Member

Dec 8, 2012
692
225
I just looked out across a 40 acre field, and I'm thinking the circular Earth theory; may be just that.

It does look awful flattish.

18. ### Lightfire Thread Starter Well-Known Member

Oct 5, 2010
690
21
Did I offend anyone? (That was not my intention, sorry if I did so....)

19. ### takao21203 AAC Fanatic!

Apr 28, 2012
3,682
491
Well it just does not make a difference from the viewpoint of ballistic physics.

People may read something into it but that's that.

If one wanted to talk about boy/girl attraction or put the same in some favour, one would normally use other means than a physics discussion.

You can always add in jokes such as fat people but that's not scientific.

If I maintain that flickr "retro America" group, it would typically contain a lot of Americanism, some of which by nowadays standards may look a bit weird.

20. ### Markd77 Senior Member

Sep 7, 2009
2,796
595
Not me, I was surprised how far off topic it went after my attempt at humor in post 5. I'll have to be more careful in future.

Anyway, have you got a better understanding now?