Anomalies in the strength of gravity

Yes, I know there's a bazillion threads on gravity already and my query is probably answered in one or other of them (but I can't be arsed wading through them all).

I've recently been watching Wonders of The Universe, narrated by Professor Brian Cox; specifically, the episode about gravity, 'Falling'. The full episode doesn't seem to be available yet on the web, just in case you were wondering why you can't see a video or even a link.

It's about 10 minutes into the episode and Cox is describing the gravitational effects of the Earth and Moon on each other.

Apparently, at some point in the remote past, the Earth's gravity caused waves of ROCK to ripple across the surface of the Moon; this rock 'wave' was 7 metres high!

Now this is what I don't understand (bear with me, I'm a simple soul and my knowledge of physics goes no further than "O" Level standard).

So the Earth's gravity is sufficiently powerful to cause 7 metre high waves - not of water, but of ROCK - to ripple across the Moon's surface, despite the Moon being almost a quarter of a million miles from the Earth.

And yet....

The Earth's gravity seems to have very little effect on ME - and I'm on its surface!

I know that gravitational effect is contingent on mass and proximity, and that I'm obviously much less massive than the Moon...but I'm also a quarter of a million miles closer to Earth than the Moon.

Or take something far more massive than me - a mountain, say.

If the Earth can exert such influence on lunar rock at such a distance, why doesn't it destroy mountains lying directly on its surface?

CJCrawley
The Earth's gravity seems to have very little effect on ME - and I'm on its surface!

It has a great effect on you all right: you're stuck on the surface of Earth. You can't jump back to space. Assuming of course you are not Superman.

If the Earth can exert such influence on lunar rock at such a distance, why doesn't it destroy mountains lying directly on its surface?

I believe it's because of the stress. At one point, the Moon has to decide wether it falls or go away. An orbiting moon does both. This creates a stress. Some moons are literally pulverized by that stress (that's why there's a ring around Saturn). A mountain doesn't have to decide if it falls - it already fell, and is retained by the surface of Earth.

Well, in this case I think the term "waves" is a bit misleading.

I'm pretty sure the rocks on the side of the moon that's always facing earth, with time (LOTS of time), eventually got pulled towards us, thus making them higher than the ones on the other side.

That's probably it, though I might be wrong since I'm no expert.
Really, I'm simply going by logic here...

Let's see if someone gives a better explanation, I'm curious...

MadHatter364
Well, in this case I think the term "waves" is a bit misleading.

I'm pretty sure the rocks on the side of the moon that's always facing earth, with time (LOTS of time), eventually got pulled towards us, thus making them higher than the ones on the other side.

I agree. One mustn't forget that "Falling" is just a movie. Not necessarily an encyclopedia of accuracy.

reply to post by CJCrawley

CJCrawley
So the Earth's gravity is sufficiently powerful to cause 7 metre high waves - not of water, but of ROCK - to ripple across the Moon's surface, despite the Moon being almost a quarter of a million miles from the Earth.

I haven't seen the show but you made a false assumption about the distance to the moon always being the same as it is today...it's moving away from the Earth so in the past it was much, much closer, plus right after the moon formed under some theories it was molten rock which is easier to form waves and ripples in. Don't forget the tides on Earth cause movement of huge masses of water, and it's possible the moon may not always have been tidally locked (with the same side always facing the Earth) in which case there could be large ripples due to gravity until the tidal locking took place.

curious.astro.cornell.edu...

It is not easy to estimate how far away from the Earth the Moon was when it formed, but simulations suggest is was about 3-5 times the radius of the Earth, or about 19-30 thousand km. (The Moon is currently about 384,000 km away from Earth or 3-4 thousand times further away than this.)

So imagine the moon 20 times closer than it is today and the picture changes, right? (384,000km / 19,000km = about 20x, that source's 3000-4000 is a typo or error).

It wasn't quite as close as in this video, but it gives you some idea of how the moon might have looked when it was closer (not quite this big, but big enough to reflect the fact it was 20 times closer):



On the other hand, I don't believe everything Brian Cox says.

reply to post by CJCrawley



Newtonian gravity is the sum of the forces that occur between any two objects.

You can plug in the numbers and see how much larger the force between the earth and the moon is vs. you and the Earth.
Link to the formula

Especially when the moon was much closer in the distant past.

Gravity is a billion times weaker then electrostatic force which is why a man can pick you up, separating you from the Earth. As for mountains, they are literally floating on electrostatic force. The buoyancy of granite in a medium of the Earth's mantle is greater than a bubble of air under water.

reply to post by InverseLookingGlass


...... mind = boom

Thank you all for your replies which I've starred.

I'll never be a physicist, but it's a good bit less mysterious to me now.

Very valid question.

I don't believe that science and their take on things like gravity are everything they are cracked up to be.

My belief about the Moon and Earth is based on fractals and atoms.

While you may have an atom that revolves around a nucleus, they never touch but are forever locked in an evolutionary embrace.

As for the mountainous ripples on the moon they could very simply be explained by its own techtonic forces thousands of years ago when it's own iron core and magma bloodline were alive and strong and pulsed through its previously living soul.

S & F.

GOCE, amonst other things, was actually creating a gravity map of the Earth in it's variations round the globe.

This latest mapping looks a lot more dramatic than earlier satellite *missions. So gravity varies according to where you are, where there is more or less mass, and how far away you might be from the centre of the mass, like at the Equator. The gravity on the moon varies in the same way, and the gravitational pulls between the Earth and the moon vary. So everything is dynamic, pushing and pulling throughout the Solar system, throughout the Universe.

If the Earth and Moon were then closer at a time, and if the Moon was in some stage from molten as another, (and excellent) post suggested, then Brian Cox could say that, though I think it is more of a suggestion rather than a fact. The GOCE Earth gravity map can be seen here,


www.dailymail.co.uk...



*The Grace mission is ongoing.

InverseLookingGlass
reply to post by CJCrawley

 

As for mountains, they are literally floating on electrostatic force. The buoyancy of granite in a medium of the Earth's mantle is greater than a bubble of air under water.

Mountains and isostasy and gravity seems okay, not always. Electrostatic force I dunno, sounds more like a another weak force like magnetism when not concentrated. Maybe between them and gravity one and the same thing. Those are the three things we don't really understand, the holy trinity in fact. Gravity, Electricity and Magnetism.

Double.

A so called ripple or wave could be produced when the moon is near to or at the edge of the roche limit
en.wikipedia.org...

This would induce a lot of stress on the moon, and partially the surface of the Earth also. This could drive massive internal heating in the right circumstances, however with the relative size of the Earth and Moon I think driven on the point of vulcanism is not really on the cards. It is however possible that the effect would have been more gradual and have induced a very very slowly moving wave starting from the centre of the surface facing the Earth (assuming they are already tidally locked) emanating outwards as the distance increases until finally relieving the stress after the moon moves away.

There is evidence for this fracture around the edges/boundary between the so called "light and dark side" (even if that is a misnomer i think its the easiest way of saying it)