# What stops the earth from colliding w/ moon?

1. Aug 2, 2011

### ucsdhopeful

I understand orbit and why the moon doesn't collide with the earth, but why doesn't the earth collide with the moon? Shouldn't the mutual gravitational forces attract each other, get stronger, until they eventually meet?

2. Aug 2, 2011

### diazona

Nope, the Earth orbits the moon just as the moon orbits the Earth. The center of both orbits is a point somewhere between the Earth and the moon.

3. Aug 2, 2011

### t3aguy

Their momentum keeps them going around each other.

There would have to be a force inward to interrupt this.

When the moon is at the closest point in its elliptical orbit it does pull earth closer to it, that is true, but when it goes back to its furthest point, earth moves in towards the moon again (in the other direction)

Here's a good .gif:

Last edited by a moderator: Apr 18, 2017
4. Aug 2, 2011

### nickthrop101

the moon doesnt just orbit the earth, as the center of gravity is where they orbit round, the COG is between the earth and the moon in space somewhere, the moon doesn't fly right down to the Earth as it's velocity counteracts the gravitational field, keeping it iin orbit but not colliding into us, the same happens for the earth =D hope that helps :D

5. Aug 2, 2011

### prashantjha

actually the COM of the earth-moon system lies inside the earth's mass.as both of the earth and moon pull each other with equal forces,the both wobble around itself.now this wobbling is the main reason why they both don't collide.they wobble in a way in which when the moon approches earth by the nearest distance,the earth wobbles and hence kepps it away from the moon.if this wobbling or dual revolving characteristic of a body a dual body system isn't present the it will be certain in any such system for the lighter body to collide with the heavier one..

6. Aug 2, 2011

### prashantjha

actually the COM of the earth-moon system lies inside the earth's mass.as both of the earth and moon pull each other with equal forces,the both wobble around itself.now this wobbling is the main reason why they both don't collide.they wobble in a way in which when the moon approches earth by the nearest distance,the earth wobbles and hence kepps it away from the moon.if this wobbling or dual revolving characteristic of a body a dual body system isn't present the it will be certain in any such system for the lighter body to collide with the heavier one..

we can also see this scenario as an approch by the earth to latitudly increase celestial orbid using the force of moon on earth,as shown in .gif graphic above..

7. Aug 2, 2011

### DaveC426913

That is a terrible animation. It is quite misleading - directly on the point that's being made - which is that, as the Moon orbits the Earth, so does the Earth orbit the Moon.

Better:

That centre cross is called the barycentre.

Picture a grown man in a whirling dance with a small child. The child "orbits" the grown man as they spin about a centre of rotation. But the grown man is not so much heavier than the child that he is stationary. He too orbits, but his orbit is quite small. It is smaller in a ratio related to the difference in their masses.

Last edited by a moderator: Apr 19, 2017
8. Aug 3, 2011

### prashantjha

this is what i acctually wanted to say as the main reason why both don't collide.this is the wobbling effect i was talkin' abt..

9. Aug 3, 2011

### DaveC426913

Well, frankly it was kind of inaccurate.

This is not true:
In fact, when the Moon makes its closest approach to the Earth, the Earth is likewise making its closest approach to the Moon. It's not like it "wobbles" out of the way.

10. Aug 3, 2011

### Thiago

The moon is in fact going away, about 3cm year.

There is no "wobbling effect", as the moon and the earth attracts each other the only thing that prevents they to hit is the momentum, more or less when you lance a basket ball in the basket and it almost enters and rotate a few times before goes away. (just for visualization sake not for the forces involved)

And Dave your gif was much more accurate thx for sharing.

11. Aug 4, 2011

### prashantjha

ah,dave can you please explain elaborately,in which part i'm wrong.this will also help me!!!

12. Aug 4, 2011

### mikeph

I don't think you're wrong, but the use of the word "wobble" has confused some people I think. It's a really vague word!

13. Aug 4, 2011

### DaveC426913

Mikey said it.

There is no such well-defined term as wobble. Using it does not shed any light on why the Earth does not collide with the Moon.

Though he says he doesn't think you're wrong, I'd say you're not even wrong, since it cannot be deduced what you are trying to describe to even determine whether it is correct or not.