# If the earth stops rotating

1. Sep 26, 2010

### zorro

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?

2. Sep 26, 2010

### Staff: Mentor

How can an internal force stop the earth's rotation? What do you mean?

3. Sep 26, 2010

### zorro

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.

4. Sep 26, 2010

### Staff: Mentor

That's an odd question. Can you give me a specific example?

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.

5. Sep 26, 2010

### zorro

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?

6. Sep 26, 2010

### Staff: Mentor

I assume they want you to imagine that mass of the earth somehow redistributes (due to internal forces) to change the moment of inertia.

7. Sep 26, 2010

### jgm340

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.

8. Sep 26, 2010

### Kevin_Axion

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.

9. Sep 26, 2010

### mrspeedybob

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 earths 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 earths 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 earths rotation.

10. Sep 26, 2010

### zorro

Well, that is not an internal force (from inside the earth). You are providing an external force, I suppose

11. Apr 14, 2011

### smyth

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. Apr 14, 2011

### Drakkith

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