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Vicky_h Vicky_h
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Posts: 8
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11 years ago
According to Newton's Law of Gravitation, F=GM1M2/r^2, we can calculate the gravitation of an object beyond Earth or on Earth's surface, using mass of Earth, mass of object, radius of Earth and altitude of object.
When the equation is applied to object inside Earth (beneath Earth's surface) I got gravitational force of object at the centre of the Earth = infinity. But in fact, an object at that point only experience high pressure, but no acceleration, that is no net force. Where did I get wrong?

My physics teacher said that when object is beneath Earth's surface, the mass above that altitude doesn't count towards gravitational force. That means
M1= mass of object,
M2= mass of Earth x (radius of Earth - depth of object)^3/(radius of Earth)^3,
r= radius of Earth - depth of object

But why doesn't the rest of the mass count? Can anyone explain to me?
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6 Replies
Genesis 1:31 God saw all that he had made, and it was very good

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wrote...
11 years ago
M2= mass of Earth x (radius of Earth - depth of object)^3/(radius of Earth)^3,

Where did you come up with this equation?
Biology - The only science where multiplication and division mean the same thing.
Vicky_h Author
wrote...
11 years ago
From the fomula of volume of sphere
V=(4{\pi}r^3)/3
Assuming the mass of Earth is equally distributed in the whole sphere... I don't know, is it too inaccurate?

Anyway, the idea is the mass below that altitude. It's like an inner sphere of the Earth.
Genesis 1:31 God saw all that he had made, and it was very good
wrote...
11 years ago
Right, I like your reasoning. So did the 4 disappear or something?

This is complicated.
Biology - The only science where multiplication and division mean the same thing.
Vicky_h Author
wrote...
11 years ago
Well, this is all my mind went through:
M2=mass of Earth x volume of inner sphere / volume of Earth
M2= mass of Earth x [(4{\pi}r^3)/3] / [(4{\pi}R^3)/3]
    where r=radius of inner sphere, R=radius of Earth
M2= mass of Earth x [r^3] / [R^3]
    since r=R - depth of object,
M2= mass of Earth x [(R - depth of object)^3] / [R^3]
and this is my equation. pure maths

So, can you answer my physics question now?  Grinning Face
Why only the mass of the inner sphere counts towards gravitational force?
Genesis 1:31 God saw all that he had made, and it was very good
wrote...
11 years ago
Sorry for the very late reply. Logged in after a week! What did you get for this, vicky?
Biology - The only science where multiplication and division mean the same thing.
Vicky_h Author
wrote...
11 years ago
What what did I get for this? anything needs more explanation?
Genesis 1:31 God saw all that he had made, and it was very good
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