# Tsunami increased the speed of our planet

1. Dec 31, 2004

### Sabine

i have read yesterday in one of our local newspapers that some physicians said that tsumani increased the speed of our planet (3 microseconds) and this caused a move of 2.5 cm of the earth from its orbit. though it's not a big deal but this can rarely happen. for whom have got somthg to say about that or have got info to add plz asap.

2. Dec 31, 2004

### lalbatros

Sabine,

Earthquakes redistribute the Earth’s mass on a global scale.
Eventually, after an earthquake the earth mass can be slightly closer to the earth axis of rotation than before the earthquake.

Despite these changes, the rotation momentum of the earth will not change.
This is a consequence of the Newton's law of motion.
Without external forces on a mechanical system, the rotation momentum never changes.
During an earthquake, internal forces are on display, but no external forces.

The rotation momentum that doesn't change is given by $$R=I\omega$$.

$$I$$ is the moment of inertia which represents the distribution of mass around the axis of rotations of the earth.
$$I$$ decreases when the mass gets closer to the axis of rotation.

$$\omega={2\pi}/{T}$$ is the angular rotation speed of the earth. (T is the period of rotation: roughly 86400 seconds)
$$\omega$$ increases when $$I$$ decreases, to keep $$R$$ constant.

The relation between mass distribution and angular speed is well known, see http://www.exploratorium.edu/snacks/momentum_machine.html [Broken] for a famous school experiment.
On wikipedia you can find the maths behind the conservation of rotation momentum: look here .
If you want some specialised information, http://www.ecgs.lu/pdf/jlg92/JLG92_Gross.pdf [Broken]
You can also find details on the earth rotation here .

If you can have fun with calculations, you could try to re-evaluate the 3µs you mentioned in your question.
To do this, you will need the moment of inertia of the earth, an evaluation of the mass displaced (assume a earth crust thickness of 5km and a density of 5 maybe, and some geographical area), an evaluation of the displacement in direction of the axis of rotation.

The last reference explain other sources of changes in the earth rotation speed. The 3 µs are small in comparison.

Have fun !

Last edited by a moderator: May 1, 2017
3. Dec 31, 2004

### T.Roc

yes, the other natural sources cause a loss of 20 microseconds per day (as compared to the same day on the previous year). this ads .73 sec to the unfortunate idea called "time" every year.

TRoc

4. Dec 31, 2004

### KingNothing

"speed of the planet" which speed?

5. Jan 1, 2005

### lalbatros

KingNothing,

Your question is right and it points us to the approximate way newspapers and other media usually talk about physics and science.

However, it is quite clear that only the earth rotation speed can be involved here.

Michel

6. Jan 4, 2005

### Chi Meson

Why are they talking to medical experts? You should listen to physicists!

7. Jan 5, 2005

### Galileo

Don't have much info, but heard from a friend that because of a slightly increased angular velocity an extra leap second will be introduced at the start of next year. (Or it could be a second taken away, which would be more likely.)

Last edited: Jan 5, 2005
8. Jan 7, 2005

### LURCH

Perhaps the article you read was talking about the same phenomenon spoken of in this post , although that story is talking about the Earth's rotation slowing down by 3 microseconds, as a result of the crust being raised up.

The newspaper article cited in the original post seems to be talking about a faster orbit, while the other thread discusses a slower axial rotation. It would not surprise me if both of these articles were talking about the same phenomenon, and getting the fats mixed up.