The Naked Scientists

The Naked Scientists Forum

Author Topic: If the Moon causes tides, why wouldn't we feel it's pull at the centre of the Earth?  (Read 4174 times)

Aaron Thomas

  • Guest
Aaron Thomas asked the Naked Scientists:
   Dear Chris,

Thank you for answering my question in your Astronomy Podcast.

I would like to pick your brain again though, in relation to your answers included in the show on the 25/11/10.

If the fact that we are in so called free-fall orbit around the sun would indicate that this area of zero gravity is not affected by the gravity of the moon or sun then;

Why do we have tides on the surface of the planet where gravity is at full strength?  Is this not effected by the pull of the moons gravity?  If the moon can exert such an influence on the surface of the Earth then would it also do something to the huge area within the Earth that is less effected by gravity?

Aaron Thomas

What do you think?
« Last Edit: 27/11/2010 20:30:04 by _system »


 

Offline tbarron

  • Jr. Member
  • **
  • Posts: 22
    • View Profile
Aaron Thomas asked the Naked Scientists:
   Dear Chris,

Thank you for answering my question in your Astronomy Podcast [nofollow].

I would like to pick your brain again though, in relation to your answers included in the show on the 25/11/10.

If the fact that we are in so called free-fall orbit around the sun would indicate that this area of zero gravity is not affected by the gravity of the moon or sun then;

But it doesn't. The fact that we're in free-fall orbit around the sun means that if we can get off the Earth and travel in a similar orbit around the Sun, it will feel to us as if we're in zero gravity.

That's very different from the idea that there's an area of apparent zero gravity at the center of the Earth because the gravitational pull from the particles on one side of the Earth cancel the pull from the particles on the other size.

Perhaps a better way to say it is that "zero gravity" doesn't exist anywhere in our universe. All protons, neutrons, and electrons exert gravitational force on all the others all the time. There are places where it may appear that the force of gravity is very weak because the forces in different directions are balanced so that they cancel each other out. Also, the gravitational force between particles that are very far apart is very weak because its strength drops with the square of the distance. Twice the distance means one quarter the force.

Quote
Why do we have tides on the surface of the planet where gravity is at full strength?

Gravity is full strength everywhere. It's not stronger here and weaker there. When the force in different directions is balanced so it cancels out, it may appear very weak. I think we have tides throughout the planet. I think the bottom of the ocean feels the Moon's tidal pull and so does the molten iron at the core.

Quote
Is this not effected by the pull of the moons gravity?  If the moon can exert such an influence on the surface of the Earth then would it also do something to the huge area within the Earth that is less effected by gravity?

It's not less affected by gravity. You're correct that the moon's influence has an effect both at the surface and beneath it.

That's what I think.
 

Offline Pikaia

  • Full Member
  • ***
  • Posts: 81
    • View Profile
The tides are caused by the fact that the moon's gravity is slightly stronger on the side of the Earth which is nearer to the moon, compared to the side which is furthest away. The tides are therefore caused by variations in the moon's gravity over the Earth as a whole, so it is rather meaningless to talk about the tidal force at a particular point.
 

Offline syhprum

  • Neilep Level Member
  • ******
  • Posts: 3813
  • Thanked: 19 times
    • View Profile
Tbaron

I think the consensus of opinion is that the core of the Earth is solid Iron surrounded by a layer of molten Iron.
 

Offline tbarron

  • Jr. Member
  • **
  • Posts: 22
    • View Profile
Tbaron

I think the consensus of opinion is that the core of the Earth is solid Iron surrounded by a layer of molten Iron.

I didn't know that. Thanks for straightening me out.
 

Offline yor_on

  • Naked Science Forum GOD!
  • *******
  • Posts: 11978
  • Thanked: 4 times
  • (Ah, yes:) *a table is always good to hide under*
    • View Profile
The range of gravity.

"The rule is as follows: every time that you get twice as far away from the center of the Earth, the gravity is only a quarter as strong. The radius of the Earth is about 6400 km so if you are 6400 km above the ground (and so twice 6400 km from the center of the Earth), then the gravity is only a quarter as strong as it was at the surface. The gravity of other things also continues forever and declines with the square of the distance. "

==
Reading it wrong, needed tea.
Or can I blame it on something else?
Awhh. It's perfectly correct.

Anyway, gravity is the universes 'Safety net' and perfectly adapted to the universes 'size''. What one might wonder about is the expansion. Gravity propagates with the speed of light and just as some light can't 'make it' all the way to Earth as the universe expands in every 'point' and so builds a constantly growing 'cleft' between that light and us so it must be with those 'gravity waves'. Is there any one having a number for how fast a square meter of new 'space' is supposed to be created. It should be countable? As we have some speed to the expansion? And then compare that to the light speed in that meter, getting a comparison. Just out of curiosity :)
« Last Edit: 03/12/2010 09:57:46 by yor_on »
 

Offline Bengt

  • Full Member
  • ***
  • Posts: 53
    • View Profile
The mechanism of Gravity, and why it's everywhere.
For a more in-depth hypothesis, see:
http://www.dipole.se
Bengt 
 

Offline Foolosophy

  • Sr. Member
  • ****
  • Posts: 218
    • View Profile
Gravity is full strength everywhere. It's not stronger here and weaker there.

......interesting statement - care to elaborate?
« Last Edit: 03/12/2010 16:17:16 by Foolosophy »
 

Offline yor_on

  • Naked Science Forum GOD!
  • *******
  • Posts: 11978
  • Thanked: 4 times
  • (Ah, yes:) *a table is always good to hide under*
    • View Profile
Inside the Earth huh :) I think of it as geodesics, and gravity waves 'deforms' the matter as it passes through, so yes, gravity influence inside matter although how the moon would do so? Heavy mathematics methinks, it depends on distance, mass, rotation, not that the moon rotates versus Earth,but it do rotate and that should be an gravitational influence too? Tricky one I think.
 

The Naked Scientists Forum


 

SMF 2.0.10 | SMF © 2015, Simple Machines
SMFAds for Free Forums