Moon and stuff

Howardnp

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Could someone either..... explain or point me to a web site which adequately covers in reasonably simple terms why when there is a bulge of water on one side of the earth there is another on the opposite side.

If this has been covered before, apologies.
Thanks
 

MarkV

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Surly if it is just caused by centrifugal force there would be a band of water bulging all around all of the time. I understood that the combination of earth and moon have a centre of gravity displaced towards the moon, it is the combination of this off centre gravity and the centrifugal force that causes the water to bulge. On the moon side it bulges because there is increesed gravity and centrifugal force acting together, towards the sides there is an equilibrium point between gravity and centrifugal force, and finally away from the moon there is less gravity so centrifugal force pushing out causes another bulge.
 

Das_Boot

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If you read the question he has taken into account the gravitaitonal effect of the moon and was asking what other effect was causing the bulge. I know there are other issues but the simple answer is centrifugal force.
 

paulskent

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There is a brilliant animation on this provided by the RNLI but the only place I can find it is here

web page

If you go to the bit about "how tides work" (in the animation screen)

click on
coastal deep water module
then
how tides work

I think you will find that it helps. I did.

PK.
 

Das_Boot

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It is a reasonable explanation however instead of saying that the moon is pulling the earth towards it and therefore causing a bulge.
The explantion I was taught is
The earth it trying to escape the pull of the moon (angular velocity/centrafugal force)
The solid part of the earth is denser than the wet part. The solid part is therefore held more firmly therefore the water is able to escape a little more and bulges where the moons gravitational force is least.
 

ashanta

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Its caused by the gravity pull of both the sun and the moon. thats why we have neaps and springs. Springs when the moon ans sun's gravitational pull is in line, neaps when moon is at right angles to the sun with earth as the axix.

Regards.

Peter.
 

AliM

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I understood that the moon attracts the water on the near side of the earth, causing a bulge. The moon also attracts the earth, less than the water the near side, but more than the water the far side, so that gives the bulge the far side of the earth. If you do the maths, you find that the attraction of the water (or anything else) to the sun is greater than to the moon. So why is the moon the dominant tidal effect? I'll answer the question myself, having figured it out - it is because it is the difference between the force exerted by the moon on the water and that on the earth which is the important thing, not the actual magnitude of the force.

The cetrifugal force argument doesn't work, because it's constant at all times, although it does lead to the sea level being higher (i.e. further from the centre of the earth) at the equator than the poles, and the slightly flattened shape of the earth in the first place.
 

Das_Boot

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You are labouring under a misconception The cetrifugal force I and the RNLI web site are talking about is not caused by the rotation of the earth it is caused by the orbit of the Earth and moon.

Have a look at the web site before making a judgment.
 

richardabeattie

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This is not an attempt to answer the question but to demonstrate pedantry. There is no such thing as centrifugal force. Objects move in straight lines at constant speed unless accelerated/deflected by external force. The Earth exerts a centripetal force pulling the moon from a straight line path down into an orbit. The force you feel when you whirl a brick on a rope is the force you are exerting on the brick. Not a force being imposed by the innocent brick. Centrifual force is just an apparent force.
 

Das_Boot

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It is difficult to make an explanation of the force exerted by the moon and earth upon each other using a string and a brick. They are different forces. One is gravitational the other is mechanical. Gravitation is a two way street. Perhaps using the word centrifugal is incorrect but the concept is not. Get onto the RNLI Website and complain.
 

seaesta

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Howard,

I went into this in a lot of detail soem years ago. My understanding after quizzing several authorities is as follows
1. There is one "bulge" (high tide) attracted to the moon etc
2. The second bulge is caused by the earth being attracted towards the moon by its gravitational pull. Imagine a tennis ball covered in liquid. If you held this and rapidly pulled (accelerated) it towards you the liquid on the far side of the ball would bulge up as its inertia resists the acceleration. The acceleration of the earth under the effect of the moon's gravity causes a similar action to that of the above balland hence the second tide.
This also explains why the morning tides on two successive days are more like each other than the intervening evening tides.
This may be a load of Clap but its the best I can come up with after a lot of work
Martin
 

Howardnp

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Thank you one and all.... I'll go for a lie down now... but what holds me to the bed...is it gravity or a centrapetal force because the earth's rotation is trying to throw me off...... don't answer! :) /forums/images/graemlins/smile.gif
 
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