# Planets and gravitation

1. Apr 2, 2005

### KingOfTwilight

We all know that moon is under the influence of Earth gravitational force and vice versa. This force causes the circular motion of Moon (and Earth as well). Earth pulls Moon with a force F and Moon pulls Earth with the same force. This has occupied me a long time:
Will Moon eventually fall down to Earth (collision is maybe a better term)?

If not, how should I try to understand gravitation? At the moment I use this kind of example:
Earth is like me swinging an object around me, the object is attached to a string and the string is like gravitation, it holds the object on orbit. Still, this causes some problems to me. When I swing that object I'll lose energy (or do I?) - I can't swing it forever. However, Earth doesn't loose any energy "swinging" the Moon, does it?
What is wrong in my model?

2. Apr 2, 2005

### neutrino

What the string does is to keep the object from going in a straight line - in a direction tangential to its orbit/trajectory (the path the object would have taken in the absence of the tension in the string). Although you may lose energy in the physiological sense of the term and hence may losen your grip on the string, in the case of the Earth, it is not so. Everything that has mass gravitates. As long the there is the pull of gravity to keep the moon in place (strictly speaking, the moon does not maintain a perfect orbit and is slowly moving away), it's not going to collide with the Earth.

Hope this helps.

3. Apr 2, 2005

### strid

when you swing the object arounf you it will be slowed down due to friction (in the string as well as air resistance). In order to keep it rotating you will have to add energy into it all the time which will obvioulsy take energy from you and hence you can do it forever...

the moon on the other hand does have extremly low friction (not string holding it :)... nor very much fluid resistance if any). So it dont need to be acclerated to keep roatating unlike your object so the moon doesnt need any eergy from the earth...

but on the other hand the gravity doesnt only pull the moon towards the earth... it also pulls the earth towards the moon.. so the earth moves a litlte towards the moon. But as the moon rotates around the earth and affects it from all sides it shouldnt move eearth very much...

4. Apr 2, 2005

### KingOfTwilight

Thanks, so my "string model" is correct, friction just takes too big effect and that's why it fails.
Strid: Yes. Both Earth and Moon go around one center of mass but Earth is so much bigger that the mass center isn't very far away from Earth's center.