# Gravity help

1. Sep 26, 2004

### tntcoder

Hi,

New to this forum and my physics isnt great, and i need to improve a hell of alot for this years exams :tongue2:.

Firstly,

Could someone explain to me how the earth sits in a static orbit with the sun, in that they dont move closer to each other.

As i thought until tonight, that two objects with mass will attract each other, something with the universal gravitational constant. So therefore if the earth is in a eliptical orbit with the sun, with centripetal motion why dosnt the suns gravity pull and the fact that all particles attract other particles pull the earth closer to the sun?

I hope this is in the right forum and thanks for any help

Cheers
Jack

Last edited: Sep 26, 2004
2. Sep 26, 2004

the centripital acceleration that the earth has while it rotates the sun is a force that keeps the two from colliding into eachoter. And by the way, the earth is slowly moving towards the sun, and one day, it wwill crash into it. The motion is soo small that it will take billions of years for this to happen.

3. Sep 26, 2004

### tntcoder

Thank you, so is this the same reason why satellites crash into earth eventually? they resist earths gravity by the centripetal acceleration but not completly and eventually it gets pulled out of orbit and will collide with earth?

4. Sep 26, 2004

Staff Emeritus
Satellites are slowed down by the residual atmosphere at their height. If they were in true vacuum they wouldn't crash, because conservation of angular momentum would keep them flying unless they could transfer some of it to some other matter.

5. Sep 26, 2004

### tony873004

Wow... I've never heard that. I've always heard that the Earth will crash into the Sun because the Sun will swell up into a red giant and engulf the Earth. If the Earth is spiraling towards the Sun, what is robbing it of its orbital energy?
I would have guess the opposite. Since the Sun is constantly converting mass to energy and radiating away that energy, and blowing out a solar wind, and storms of charged particles, it would make sense that the Sun is slowly losing mass. That should cause the Earth to spiral out. The tide that the Earth pulls on the Sun should also cause it to spiral out just like our moon spirals out from Earth. But all those things combined shouldn't make much of a difference anyway.

If the Earth will spiral into the Sun, covering a distance of 1 AU over a few billion years, doesn't that imply that Earth spent the last several billion years spiraling into its current position? But since an Earth even slightly farther from the Sun would be a cold icy world in a permanent ice age, doesn't that suggest that the Earth didn't come from farther out in the solar system.

Interesting theory... You've got me curious.