Moment of inertia of the earth

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Discussion Overview

The discussion revolves around the moment of inertia of the Earth, particularly focusing on the calculated coefficient of 0.331 compared to the expected value of 0.4 for a solid sphere. Participants explore the implications of this difference and its relation to mass distribution within the Earth.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant calculated the moment of inertia of the Earth as 0.331 and questions the significance of this value compared to the solid sphere coefficient of 0.4.
  • Another participant suggests that the difference in coefficients may be due to the concentration of mass towards the center of the Earth, but expresses uncertainty about how this affects moment of inertia.
  • A participant requests clarification on how this relates to the definition of moment of inertia.
  • There is a suggestion to write out the definition of moment of inertia to clarify the relationship between mass distribution and rotational resistance.
  • One participant explains that moment of inertia measures an object's resistance to changes in its rotation rate and seeks to understand its relationship to the Earth's mass distribution.
  • A later reply discusses the dependence of the Earth's moment of inertia on its rotation rate and the effects of the equatorial bulge, suggesting a connection to gravitational and centrifugal forces.

Areas of Agreement / Disagreement

Participants express differing views on the implications of the calculated moment of inertia and its relationship to mass distribution. The discussion remains unresolved, with no consensus on the interpretation of the results or the underlying physics.

Contextual Notes

Participants reference the definition of moment of inertia and its dependence on mass distribution, but there are unresolved questions regarding the specific mathematical relationships and assumptions involved.

nns91
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Hey guys,

I just calculated the moment of the inertia of the Earth using some data. I got the coefficient as 0.331, not 0.4 as for a solid sphere. What do you guys think it means ?

I reason the different of the 2 coefficient is because more mass of the Earth concentrates toward its center. But I don't understand how that contribute to the difference in moment of inertia. How would you guy explain this ?
 
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nns91 said:
Hey guys,

I just calculated the moment of the inertia of the Earth using some data. I got the coefficient as 0.331, not 0.4 as for a solid sphere. What do you guys think it means ?

I reason the different of the 2 coefficient is because more mass of the Earth concentrates toward its center. But I don't understand how that contribute to the difference in moment of inertia. How would you guy explain this ?

From the definition of Moment of Inertia. Is this homework?
 
Nah, it's not my homework. How is this related to the definition of moment of inertia ?
 
nns91 said:
Nah, it's not my homework. How is this related to the definition of moment of inertia ?

You said in your first post above that you didn't understand why concentrating more mass at the center of the sphere would lower its moment of inertia, compared to a more uniform distribution. Write out the definition of the moment of inertia, and that should answer your question...
 
Moment of inertia is basically is the measurement of the resistance of an object to change in its rotation rate.

So what is the relationship here ?
 
Show us the equation.
 
nns91 said:
I reason the different of the 2 coefficient is because more mass of the Earth concentrates toward its center. But I don't understand how that contribute to the difference in moment of inertia. How would you guy explain this ?
The closer the mass is to the rotation axis, the lower is its velocity and therefore momentum. So you need less momentum to make it spin.
 
The moment of inertia of the Earth depends on its rotation rate (angular velocity) because the equatorial bulge of the oceans depends on the angular velocity. If the Earth's surface were 100% water, then the Earth's surface would be defined by a equipotential surface (oblate spheroid) (gravitational force - centrifugal force = constant). This might apply to the liquid core as well as the ocean surface.
 

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