moons gravity

[skilliyay]

how near to the moon would you need to be, to get drawn in by its gravity?

[jake-the-peg]

Any gravitational field gets smaller and smaller the further away you get.

In theory you could be infinitely far away and still be attracted to it if there were not other bodies that would get you first.

In the case of the moon, the Earth is nearby and there is a region of space where hte moons gravity is dominant over the Earth. this is called the moon's gravitational sphere of influence.

http://www.go.ednet.ns.ca/~larry/orbits/gravas st/gravasst.html

In the case of the moon I make it about 48,000 miles, any more than that and the Earth would get you.

Note I am ignoring the Sun's influence on this because of the problems involved in 3 body calculations.

A sim
ilar question is what speed you have to be going to escape the moon's gravity - this is 2.4 Km/s that is the escape velocity for the moon

[Peter Pedant]

This q has a long history

Newton in 1665 sat wondering this very thing - how far would an apply have to be from theearth before itdidnt fall,and came up with gravitation....

Isnt there a neutral point where the two cancel out ?


PP

[jake-the-peg]

That's right Peter, in fact there are 5 of them, they are called Lagrangian points
(inevitable wikipedia link)
http://en.wikipedia.org/wiki/Lagrangian_point

[oglecorp]

this is an incredibly simple question. the moon's gravity is 1/6 that of the earth, so you need to be [1/7 the distance from the earth to the moon] from the moon for there to be no gravity. any closer and the moon will exert a stronger pull than the earth.

[jake-the-peg]

You would have thought so wouldn't you oglecorp?

But alas the moon orbits the Earth and this complicates things as you'll see if you read the link I posted.

Laplace derrived the formula there for a two body system.

If we go even further into the real world and consider multiple bodies the obits become quite chaotic. Henri Poincare was one of the finest mathematical minds of the 20th Century and he devoted a lot of time to trying to establish whether the orbits of the planets were truely stable and was unable to do so.

As you'll see then orbital mechanics looks incredibly simple but in reality it really can be rocket science