# Earth stop to turn

1. Oct 15, 2012

### scientifico

Hello, according to the classic physics a body subjected to an acceleration must dispers energy, the earth is subjected to a centripetal and centrifugal force so why it doesn't stop to turn ?

2. Oct 15, 2012

### mikeph

I'm not sure what exact law of classical physics you're referring to but it doesn't sound correct.

On a similar subject, centripetal/centrifugal 'forces' are not physically real forces like electromagnetism or gravity. The former is actually a condition for circular motion, and the latter is the result when you analyse a problem in a rotating coordinate system.

3. Oct 16, 2012

### scientifico

probably i got the law in a wrong way, i have read that according to classical physics an electron emits energy in form of radiations because of it's acceleration and should fall into the nucleus

4. Oct 16, 2012

### mikeph

Ah, yes, it is true that an accelerating charge will emit electromagnetic radiation and lose energy in the process (a result from electrodynamics). It's a result you can derive from Maxwell's equations.

I'm not sure how this applies to a planet which is, after all, rotating and made up of charged particles (though the planet as a whole is more or less electrically neutral). Perhaps the effect cancels, or the accelerations are far too minute to produce a measurable effect?

Someone else will hopefully have a better answer!

5. Oct 16, 2012

### Staff: Mentor

The earth is extremely close to neutral, all charges are accelerated in a similar way, so we do not emit (significant) electromagnetic radiation based on our orbit around the sun.

Earth has a gravitational charge (=a mass), and emits gravitational waves. However, Earth is slow relative to the speed of light and gravity is weak, so the lost power is tiny: just ~200W. If there would be no other effects on the orbit, the radius of earth would decrease by about 1 femtometer per day. About 2 millimeters in the whole lifetime of the solar system.

6. Oct 17, 2012

### scientifico

does emission of gravitational waves involve loss of matter ?

Last edited: Oct 17, 2012
7. Oct 17, 2012

### Staff: Mentor

If you try to see the whole solar system as one object, you could say that its mass gets reduced.
However, if you consider masses in the solar system: No. It involves a loss of energy.

8. Oct 17, 2012

### mikeph

Is this loss of energy manifested as a reduction in the angular velocity of the planet? (edit- or at least a reduction in angular momentum if the radius decreases- but how does the radius decrease?)

And if so... how is angular momentum conserved? How can gravitational waves compensate for this loss of angular momentum if they're directed radially outwards?

Last edited: Oct 17, 2012
9. Oct 17, 2012

### scientifico

So why the earth radius shoud decrease 1 femtometer per day ?

10. Oct 17, 2012

### Staff: Mentor

The earth loses orbital energy.

@MikeyW: No, it is independent of the rotation of earth, the 200 W come from its orbit around the sun.

They do not have a perfect radial symmetry.

11. Oct 17, 2012

### scientifico

why if it just loses energy it loses matter too ?

12. Oct 17, 2012

### Staff: Mentor

Why should it lose matter?

13. Oct 17, 2012

### scientifico

because the redius decrease.... or maybe the matter doesn't lose and the radius became more compact

14. Oct 17, 2012

### Staff: Mentor

???

The orbital radius of earth decreases - the earth comes closer to the sun. Why do you expect any influence on its mass?

15. Oct 18, 2012

### scientifico

Oh, i tought it was the earth radius :)