What causes the tidal bulge on the far side of the Earth [Archive]

idorward

18-11-2003, 10:44

nick kay

18-11-2003, 11:50

Try (for a good explanation)

<a href="http://www.projectsandhobbies.com/Moon.htm" >http://www.projectsandhobbies.com/Moon.htm</a>

or (for a pretty picture)

<a href="http://www.sp.uconn.edu/~geo101vc/Lecture20/sld024.htm" >http://www.sp.uconn.edu/~geo101vc/Lecture20/sld024.htm</a>

John Thirlwell

18-11-2003, 11:54

OK, everyone i have asked this question to starts by saying thats obvious and it's taught very early on in diver training, but i do believe its a bit more complicated than you might first think. Any soon the people i ask say "oh, i'm not sure"... so here goes.....

Q."When the Sun and Moon are on opposite sides of the Earth you get Spring tides, when the moon is at right angles you get neaps, but why when the Sun and Moon are on the same side of the Earth do you get the tidal bulge on the far side of the Earth?"

Answers range from Centrifugal force, to Newtons Laws of motion to gravity being inversely proportional to the square of the distance.

What are your thoughts......

Ian

Ian, interesting question, did a little research on the internet, the link may help

John

matt

18-11-2003, 12:01

OK, everyone i have asked this question to starts by saying thats obvious and it's taught very early on in diver training, but i do believe its a bit more complicated than you might first think. Any soon the people i ask say "oh, i'm not sure"... so here goes.....

Q."When the Sun and Moon are on opposite sides of the Earth you get Spring tides, when the moon is at right angles you get neaps, but why when the Sun and Moon are on the same side of the Earth do you get the tidal bulge on the far side of the Earth?"

You get springs when the Sun, Moon and Earth are aligned, hence the full or new moon. You get neaps when the Moon and Sun are at right angles to the Earth.

AIUI The opposite bulge is caused by centrifugal force. Simply; the combination of gravitational forces and the centrifgal force from the Earths rotation causes the Oceans to squeeze.

Have a look here;
<a href="http://www.seafriends.org.nz/oceano/tides.htm" >http://www.seafriends.org.nz/oceano/tides.htm</a>
<a href="http://co-ops.nos.noaa.gov/restles1.html" >http://co-ops.nos.noaa.gov/restles1.html</a>

janos

18-11-2003, 20:08

Q."When the Sun and Moon are on opposite sides of the Earth you get Spring tides, when the moon is at right angles you get neaps, but why when the Sun and Moon are on the same side of the Earth do you get the tidal bulge on the far side of the Earth?"
What are your thoughts......

Speaking as a man with a degree in oceanic & atmospheric physics, (most of which has been forgotten years ago) the answer is defintely Centrifugal force. The centre of gravity of the Earth moon system is not the centre of the Earth and so there is a force exerted on Ocean which move it as far as it can from this point, ie the far side of the Earth. The near tide is caused by gravity.

My tutor used the example of Scotsman throwing a hammer to illustrate the point (metaphorically not literally!): his kilt moves outwards directly behind him.

Laters,
Janos

Ian Wigg

19-11-2003, 12:40

I remember my physics lecturer repeatedly drumming into us that it was Centripetal Force and Centrifugal Reaction. Funny how things like that stick in your brain.

PeteM

19-11-2003, 13:01

I remember my physics lecturer repeatedly drumming into us that it was Centripetal Force and Centrifugal Reaction. Funny how things like that stick in your brain.

Actually its neither :-)

Space is being bent in a centripetal direction and Newtons first law gives us an apparent reaction

Andy Wade

19-11-2003, 14:37

:=I remember my physics lecturer repeatedly drumming into us that it was Centripetal Force and Centrifugal Reaction. Funny how things like that stick in your brain.

Actually its neither :-)

Space is being bent in a centripetal direction and Newtons first law gives us an apparent reaction

Hear hear, just what I was thinking!
What was the question again?
Errrrr....
Can't we just say that it's because the earth 'knows' to have a lump of water on the other side so that it doesn't get into a nasty wobble?
A wobbly earth, scary stuff. We just don't want to go there.
The water is actually trying to leave and the earths gravity is holding on to it, some of it gets missed though, and it runs around the other side of the earth, and tries to get out the back door.
Simple.
;-)

Ian Wigg

20-11-2003, 14:00

Ian Wigg

20-11-2003, 14:29

This might help (then again it might not!!)

janos

20-11-2003, 19:54

IIRC you can make any force 'real' if you choose a suitable frame of reference.

Laters,
Janos

PS - Thanks for the drysuit (it was you wasn't it?) - it didn't fit but we managed to sell it on for the same price.

PeteM

21-11-2003, 10:09

IIRC you can make any force 'real' if you choose a suitable frame of reference.

Not sure about that, a force is a force, however you can make any speed/direction real by picking the right frame of reference, for instance:-

Who is travelling faster me on the motorist driving past my office window?

From the perspective of earth surface the motorist because he is moving and I'm not.

From the perspective of the centre of the earth; it depends if he is travelling east or west, east I'm faster because I am travelling at the earths rotational speed and he is traveling slightly slower.

Its a bit early in the day to start working it out from the perspective of the sun or the centre of the universe.

Pete

janos

21-11-2003, 20:28

:=IIRC you can make any force 'real' if you choose a suitable frame of reference.
:=

Not sure about that, a force is a force, however you can make any speed/direction real by picking the right frame of reference

I think the theory then goes that if a speed is 'real' then a change in velocity is also real and so consequently a force is real.

But enough! My head is starting to hurt already! It's been years since I did this sort of thing and it scares me how much I've forgotten. I'm going to bring the topic slightly back to something more practical though. Perhaps there are some Physicists who have better memories (or with less rotted brains) who can help me?

How long does it take two tanks to equalise mixes through a manifold? I've tried to work it out using Kinetic Theory, but I've forgotten it all and my textbooks are in the loft. I used to be able to derive effusion through a small hole with my eyes shut, but sadly now can't even remember the answer

All donations to the brain restoration fund gratefully accepted.

Laters,
Janos

Ian Wigg

24-11-2003, 14:38

:=:=IIRC you can make any force 'real' if you choose a suitable frame of reference.
:=:=
:=
:=Not sure about that, a force is a force, however you can make any speed/direction real by picking the right frame of reference

I think the theory then goes that if a speed is 'real' then a change in velocity is also real and so consequently a force is real.

But enough! My head is starting to hurt already! It's been years since I did this sort of thing and it scares me how much I've forgotten. I'm going to bring the topic slightly back to something more practical though. Perhaps there are some Physicists who have better memories (or with less rotted brains) who can help me?

How long does it take two tanks to equalise mixes through a manifold? I've tried to work it out using Kinetic Theory, but I've forgotten it all and my textbooks are in the loft. I used to be able to derive effusion through a small hole with my eyes shut, but sadly now can't even remember the answer

All donations to the brain restoration fund gratefully accepted.

Laters,
Janos

Taking into account Shroedinger's cat paradox there's no guarantee that there's anything in your tanks anyway.

Ian

PeteM

25-11-2003, 09:45

:=Not sure about that, a force is a force, however you can make any speed/direction real by picking the right frame of reference

I think the theory then goes that if a speed is 'real' then a change in velocity is also real and so consequently a force is real.

But a change in velocity is always a change in velocity, what it started as and what it finished as is dependent on view point but the change is not. Example: You're in a coach, you throw a ball from one end to the other, it takes the same force to throw it which ever direction you throw it and what ever speed the coach is running despite the fact that the velocity of the ball will be totally different depending on the coaches speed across the ground and whether you are throwing it front to back or back to front

Pete

Bob Fry

02-12-2003, 01:46

Now what's REALLY scary is that the Earth's crust does the same thing. By about six inches, I believe. It's an unsettling thought for me at least. Mind how you go, don't step on the cracks!

OK, everyone i have asked this question to starts by saying thats obvious and it's taught very early on in diver training, but i do believe its a bit more complicated than you might first think. Any soon the people i ask say "oh, i'm not sure"... so here goes.....

Q."When the Sun and Moon are on opposite sides of the Earth you get Spring tides, when the moon is at right angles you get neaps, but why when the Sun and Moon are on the same side of the Earth do you get the tidal bulge on the far side of the Earth?"

Answers range from Centrifugal force, to Newtons Laws of motion to gravity being inversely proportional to the square of the distance.

What are your thoughts......

Ian

Adrian Kelland

02-12-2003, 14:19

Now what's REALLY scary is that the Earth's crust does the same thing. By about six inches, I believe. It's an unsettling thought for me at least. Mind how you go, don't step on the cracks!

Well the moon and sun are only responsible for about 18" of tide, and the moon does most of that. The land constrictions are responsible for most of the height changes.

And no cracks!

john bache

21-12-2003, 23:16

If a coach is in constant straight line motion ( or at rest), you have an inertial frame of reference inside the coach, and Newton's laws work OK inside the coach without you knowing it's velocity or direction. But if the coach driver wants a laugh and drives around a roundabout all day then you can still have the inside of your coach as a frame of reference but now it's a non-inertial frame of reference with an apparently weird force acting sideways .... sometimes called centrifugal force. If you take a frame of reference outside the coach then the friction of it's tyres on the road pulling the coach round into a circular path can be called a centripetal force.

I've not time to read much more about it as the tide is coming in !

John.