# B Why the Earth moves around the Sun forever and ever and ever?

1. Sep 23, 2016

### parshyaa

why earth moves around sun forever and ever and ever.
• Earth wants to move in straight Line forever and ever and ever(as there is no ext. Force)but due to massive sun space get bends(according to GR) and moving earth starts moves in circle(or not perfect circle) , as it was moving in straight line forever and ever and ever it continues so in circle
I want the main answer for why earth moves forever and ever and ever,why shape do not makes any effect .​

2. Sep 23, 2016

### Staff: Mentor

Newton's first law.

3. Sep 23, 2016

### parshyaa

You mean my reason is correct

4. Sep 23, 2016

### parshyaa

You mean, there is no effect of shape occured by sun in space and time, earliar it was moving in st. Line forever and ever and ever which it still continues in circle also

5. Sep 23, 2016

### Staff: Mentor

Is this homework? The wording implies two different questions at two different levels. The "forever and ever and ever" part implies to me that that is the main question, not the "why earth moves around sun" part. So I was answering that part.

The level of the second part imlpies a lower level for the first part: I would not use General Relativity as an answer to why the earth is in orbit, I'd use Newton's law of gravity. And the two parts become very related to each other when using Newton's Laws of gravity and motion.

6. Sep 23, 2016

### parshyaa

No its not a HW question , I just wanted to know that newtons first law say that object without any force will be in motion in a straight Line forever and ever, so is it true when its path becomes circular due to bending of space by sun .

7. Sep 23, 2016

### sophiecentaur

It is essential to realise the nothing inanimate "wants" to do anything. Anthropomorphism can get in the way of understanding things. You can choose between a Newtonian explanation of orbits or a GR explanation - each one has its place.
Things will go on 'for ever and ever' unless there is some energy loss involved. The loss of energy in the Earth / Sun system is very very low and the orbit will decay by an insignificant degree in our time scale (undetectable until quite recently). But there are loss mechanisms even in astronomical processes and everything changes in time.

8. Sep 23, 2016

### pixel

I'm not sure what "shape" you are referring to here. Shape of the orbit?

If space were a true vacuum, then it would move forever as there are no dissipative forces. In reality, since space is not a true vacuum, over a long time the earth's orbit would change due to collisions with particles in its path around the sun. That would be a very long time.

9. Sep 23, 2016

### parshyaa

yes the circular orbit , newtons law says that it will move forever and ever and ever in a straight line but here the path or orbit becomes circular then also earth moves forever and ever and ever,why??

10. Sep 23, 2016

### Staff: Mentor

The shape is due to gravity pulling it into a circular (elliptical) path. The "forever and ever" is due to there being no force opposing the motion (if we stick to circular).

11. Sep 23, 2016

### parshyaa

You mean that there is no effect of shape circular(eleptical) on newtons first law , which says that if there is no force object will move in straight line but here its path became circular.

12. Sep 23, 2016

### pixel

There is a force here - gravity - so the earth doesn't move in a straight line.

13. Sep 23, 2016

### pixel

Why have to stick to circular?

14. Sep 23, 2016

### parshyaa

But gravity is not a force its because of shape

15. Sep 23, 2016

### Khashishi

Not forever. Eventually the Sun will engulf the Earth. But that won't be for a few billion years.

16. Sep 23, 2016

### parshyaa

Now thats what I was searching , thanks khashishi

17. Sep 23, 2016

### Staff: Mentor

Only if you are using general relativity, and there's no reason to do that here.

Newton developed his theory of gravity, in which gravity is a force determined by $F=Gm_1m_2/r^2$ late in the seventeenth century. Two hundred and fifty years later Einstein developed his theory of gravity, in which gravity is not a force but instead the result of spacetime curvature described by $G_{\mu\nu}=8\pi{T}_{\mu\nu}$.

It's easy to look at that history and say that Einstein showed that Newton was wrong and gravity isn't a force - but that would be wrong. Gravity is what is, and the universe does what it does, and the two theories are two different mathematical descriptions of that behavior. Although they arrive at this result in different ways, both theories predict that the earth will move in an elliptical orbit that is very close to circular. In both theories the underlying physical explanation for why the earth can keep moving is the same: there is no friction acting on it in any significant way.

It is fair to say that Einstein's theory is better than Newton's, because it makes more accurate predictions over a wider range of physical problems than Newton's. But saying that Einstein's theory s better is not the same as saying that Newton's theory is wrong; it works just fine for planetary motions. Indeed, I'll venture a stronger statement: it is impossible to learn and understand Einstein's theory if you do not already have a solid understanding of Newton's theory.