Why Doesn't Conservation of Momentum Apply if Earth Stops Rotating?

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Discussion Overview

The discussion revolves around the implications of the Earth's rotation stopping suddenly, particularly focusing on the conservation of momentum and angular momentum. Participants explore theoretical scenarios, internal and external forces, and the effects on the Earth's orbit and moment of inertia.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification
  • Mathematical reasoning

Main Points Raised

  • Some participants question how an internal force could stop the Earth's rotation, seeking clarification on the nature of such forces.
  • Others discuss the concept of angular momentum, noting that it would be conserved if only internal forces were at play, but challenge the necessity of this concept when the speed is already stated to decrease.
  • There are inquiries about how the moment of inertia of the Earth could change suddenly and what implications this would have on the duration of the day.
  • One participant suggests that for the Earth to stop rotating, all its mass would need to move infinitely far from its center, while another posits that pulling mass into a single point would cause infinite rotation.
  • A later reply introduces the idea that if the Earth stopped rotating, it could lead to atmospheric ionization and loss of magnetic field lines, although this does not directly address the original question.
  • One participant proposes a hypothetical scenario involving a motor attached to a mass that could theoretically alter the Earth's rotation, but this is challenged as not being an internal force.
  • Some participants express curiosity about the effects on Earth's orbit if it were to stop rotating, with one suggesting that it would not significantly affect its orbit.

Areas of Agreement / Disagreement

Participants generally do not reach a consensus on the mechanics of stopping the Earth's rotation, with multiple competing views on the nature of forces involved and the implications for angular momentum and orbital dynamics.

Contextual Notes

Limitations include assumptions about the nature of forces, the definition of internal versus external forces, and the hypothetical scenarios presented without resolving the underlying physics.

zorro
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Suppose the Earth stops rotating suddenly ( say due to some large internal force ). If we consider the whole Earth as a system and apply conservation of momentum:

I1w1=I1(0)

This equation does not work. Why is it so?
 
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Abdul Quadeer said:
Suppose the Earth stops rotating suddenly ( say due to some large internal force ).
How can an internal force stop the Earth's rotation? What do you mean?
 
I have seen many questions like 'Suppose the speed of rotation of Earth decreases by some internal force, how would it affect the duration of day' etc
In these questions we use angular momentum concept.
 
Abdul Quadeer said:
I have seen many questions like 'Suppose the speed of rotation of Earth decreases by some internal force, how would it affect the duration of day' etc
That's an odd question. Can you give me a specific example?

In these questions we use angular momentum concept.
It's certainly true that angular momentum will be conserved if the only forces are internal. But why would you need to use that fact--you're told that the speed decreases.
 
I'm Sorry,
Questions are not like that.
I read that if the moment of inertia of the Earth decreases/increases suddenly, then how will it affect the duration of the day.
How can moment of inertia change suddenly?
 
Abdul Quadeer said:
I read that if the moment of inertia of the Earth decreases/increases suddenly, then how will it affect the duration of the day.
How can moment of inertia change suddenly?
I assume they want you to imagine that mass of the Earth somehow redistributes (due to internal forces) to change the moment of inertia.
 
In order for the Earth to stop rotating entirely, all its mass would need to go infinitely far away from the center of the earth.

In order for the Earth to start rotating infinitely fast, all its mass would need to be pulled into a single point at the center.
 
If the Earth stops rotating highly energetic plasma will ionize the atmosphere, strip it off and bombard the Earth with extreme radiation (non-rotating, liquid iron causes a loss of magnetic field lines). I know this doesn't answer the question but it's still interesting.
 
Doc Al said:
How can an internal force stop the Earth's rotation? What do you mean?

suppose a motor was attached to a mass such that the axis of the masses rotation coincided with the axis of the Earth then you could alter the speed of the Earth's rotation by changing the rotational speed of the mass. Total angular momentum would be conserved. If you accelerated the mass in the same direction as Earth's rotation then the reaction torque would slow the earth. Given a large enough mass and a powerful enough motor you could (in theory) stop the Earth's rotation.
 
  • #10
Well, that is not an internal force (from inside the earth). You are providing an external force, I suppose
 
  • #11
i was looking for an answer to this question " what happens to Earth if suddenly stops rotating".
and i don't care about humans, trees, mountains flying away at very high speed, what I'm curios about is how much the orbit of Earth is affected. Let's say I'm a very , very large entity, with fingers, and that I'm putting my finger on Earth, in such way that i forbid him to revolve anymore. In this case, would Earth fly away from the solar system?
 
  • #12
smyth said:
i was looking for an answer to this question " what happens to Earth if suddenly stops rotating".
and i don't care about humans, trees, mountains flying away at very high speed, what I'm curios about is how much the orbit of Earth is affected. Let's say I'm a very , very large entity, with fingers, and that I'm putting my finger on Earth, in such way that i forbid him to revolve anymore. In this case, would Earth fly away from the solar system?

No. It would still go in more or less in the same orbit.
 

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