# Gravity force of planets

1. Jul 4, 2011

### Redi

Why do specific object movement is conditioned by the gravity force? Why the moon is pulled by Earth and doesnt move freely? Why the planets dont move in different way but obey to the gravitacional force of Sun? Everybody knows that Earth moves around the sun for 1 year and travels for hundreds of kilometres.The moon moves together with the Earth in this 1 year trip. What is the force that comands the Moon to stay with earth and don`t leave it? And does the gravitacional force of Sun affect Moon as same as it affects Earth? Moon and Earth pull each-other and move under the affect of gravity of Sun . Is there a force that keeps them in a specific distance from each-other or they will pull each-other for a long time until they maybe crash? I think gravity of Sun made Moon come closer to Earth and than caused this year catrastophy in Japan?

2. Jul 4, 2011

### Janus

Staff Emeritus
Gravity is the attraction matter to matter. for two objects it is proportional to the product of the objects' mass divided by the distance between the objects' centers.

The Earth orbits the Sun and the Moon the Earth because of a balance between the gravitational attraction and the the tendency for them to travel in straight line due to their velocities. The Earth, left to itself would fly off in a straight line at -30 km/s. The pull from the Sun curves this path into an orbit. The same goes for the Earth and Moon.
The exact details of these orbits is fairly complex and a field of study in of itself.

The Sun does interact with the Moon's orbit as do, to a much smaller degree, other planets in the solar system. This tends to stretch and compress the Moon's orbit slightly. However, this is a fairly small effect and is dwarfed by the fact that the Moon's orbit is naturally elliptical to start with and its distance from the Earth varies over the course of a month. This causes a variation in the tides.

Tidal interaction between the Earth and Moon is actually increasing the distance between the Earth and Moon, so there is no fear that the Moon will ever crash into the Earth.

In short, the Moon hasn't done anything beyond what it has been doing naturally for millions of years, nor do we have any reason to expect it to in the future. So, no, the Moon did not have anything to do with the Japan Earthquake.

3. Jul 5, 2011

Two forces

1. Gravity = pulling at each other
2. Centrifugal force = trying to move away from each other

Keeps them at a safe distance.

BTW
The moon is slowly moving away from the earth. So in the far future it will be significantly farther from earth than it is now.

4. Jul 6, 2011