What is the relationship between Earth's rotation and its mass?

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Discussion Overview

The discussion revolves around the relationship between Earth's rotation and its mass, particularly in the context of mass loss and redistribution. Participants explore theoretical implications of mass changes on rotational speed, considering both closed and open system perspectives, as well as the effects of mass distribution.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Exploratory

Main Points Raised

  • One participant argues that Earth is a closed system regarding mass, suggesting that losing weight does not affect its rotational speed since the mass remains within the system.
  • Another participant counters that Earth is not a closed system because it receives sunlight and loses heat, which could affect its overall mass.
  • A participant introduces the idea of mass distribution, questioning whether the rotation changes if mass is redistributed, such as through sweating or consuming food.
  • It is noted that in a closed system, angular momentum is conserved, implying that if mass moves closer to the axis of rotation, the spin rate could increase.
  • One participant mentions that mass lost through biological processes, such as calories burned, contributes to the overall mass change of the planet.
  • Another point raised is that Earth radiates heat due to its temperature, independent of biological activity, and that this radiation affects its thermal equilibrium.

Areas of Agreement / Disagreement

Participants express differing views on whether Earth can be considered a closed system and how mass loss or redistribution affects its rotation. There is no consensus on these points, and multiple competing views remain.

Contextual Notes

Participants acknowledge the complexity of the system, including factors like mass distribution and energy exchange with the environment, which complicate the discussion of Earth's rotation and mass.

kernelpenguin
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On the UserFriendly forums I saw a message a few days ago from two people that had lost some weight. It went something like: "If you've noticed that the Earth is spinning faster, it's because X and Y have lost N pounds!"

Okay, logically, an object should start spinning faster if it sheds some mass, but does it, in this case, shed any mass at all?

I got into a debate about this with my friend.

My point of view is that the Earth is a closed system when it comes to particles with mass. Even if they did lose N pounds, that still hasn't left Earth or its atmosphere, so Earth should be spinning at exactly the same speed.

His point of view was that Earth is not a closed system since it gets sunlight and sheds heat radiation. But these shouldn't affect mass, should they? Then he went on saying that some of it is deposited in the atmosphere and thus doesn't contribute to Earth's rotation.

It's a fun little problem, methinks. Anyone care to cast their opinions? :)
 
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Don't forget to think in terms of mass distribution too, not just total mass! Is the rotation of the Earth the same once someone has sweated off 1 pound? Or has eaten a carrot that was underground?
 
This is like the stereotypical ice sk8er problem, but instead of arms and legs moving in toward the center, is it not(X and Y)
 
In a closed system, it is not the spin that is conserved, but the angular momentum. Therefore, if mass is re-distributed to be closer to the axis, the spin rate will increase. Like a skater pulling his arms in.
 
Earth is not a closed system, and does indeed receive sunlight and radiate heat. This will have an effect on the overall mass of the planet because the lost weight was lost as calories burned by biological activity. This mass was converted to heat, which can leave the Earth and its atmosphere.
 
Earth also radiates just because of its temperature, as anybody does. It doesn't require biology to do that. Indeed the Earth can be seen as a system in near thermal equilibrium, losing as much heat by radiation as it gains from insolation. If the greenhouse effect prevents heat from being radiated, it will reach equilibrium at a higher temperature, as has happened to Venus.
 

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