wu :: forums - Print Page


    
      
        wu :: forums
        (http://www.ocf.berkeley.edu/~wwu/cgi-bin/yabb/YaBB.cgi)
      

        riddles >> easy >> why ocean doesn't dry up?
        
(Message started by: Rejeev on Oct 4^th, 2004, 6:36am)

Title: why ocean doesn't dry up?
Post by Rejeev on Oct 4^th, 2004, 6:36am

Shape of Earth is spheroid. That means at many points [consider a point on ocean] direction towards the centre is not perpenticular to the surface. why then from the surface, water does not flow towards the inclination?

Title: Re: why ocean doesn't dry up?
Post by harpanet on Oct 4^th, 2004, 2:15pm

My little pencil drawing indicates that the water would tend to gather at the poles. So my initial thought is :::[hide]that the rotation of the earth would counteract that tendency[/hide]:::.

Another possibility is that :::[hide]the effect is small and is overwhelmed by other elements in the system (e.g. topology of the ocean beds, moon's gravitational attraction[/hide]:::.

Title: Re: why ocean doesn't dry up?
Post by John_Gaughan on Oct 5^th, 2004, 7:15am

Try going to a place like Panama that is only a few miles wide between two oceans. The Atlantic and Pacific are not often at the same level there. Why is this? Earth's rotation, tides, and non-uniform gravity due to the ellipsoid shape of Earth. The oceans are constantly moving. Not only the water particles themselves, but the levels rise and fall in various places as they are pulled around the Earth by various forces. Even hurricanes can affect this -- Ivan had 50+ foot waves in the eye, and caused water surges over 10 feet tall hundreds of miles away, with 20+ foot surges in his path.

Title: Re: why ocean doesn't dry up?
Post by Icarus on Oct 7^th, 2004, 8:24pm

The other reason is also the answer to the question "why is the Earth a spheroid instead of a sphere?" On the scale of the full earth, rock is essentially fluid. This means that were the earth no subjected to additional forces, gravity would pull it into an essentially spherical shape. That it is elliptical in the first place tells you that there are other forces at work, and the clear candidate is the centrifugal force from the earth's rotation. The surface of a fluid subjected to forces will lie along a surface of equipotential. For both the rock and the water of Earth, the combination of centrifugal force from the spin and centripetal force from gravity makes the equipotential surfaces spheriodal.

Title: Re: why ocean doesn't dry up?
Post by rmsgrey on Oct 8^th, 2004, 2:59am

There's an interesting possibility for a habitable world (an example of which forms the setting of Robert L Forward's Rocheworld) which consists of two planets of roughly equal mass, orbiting their common center, which take on the shape of two teardrops, pointing towards each other, with local gravity always perpendicular to the surface (except for minor irregularities like mountains). Between the two is a point of 0-gravity, which is "above" both planets, but "below" the surrounding space. In the novel, one body is slightly more massive than the other, so "lower" and has accumulated practically the entire supply of water for both bodies. The difference in "height" is small enough that both bodies share an atmosphere.

Title: Re: why ocean doesn't dry up?
Post by John_Gaughan on Oct 8^th, 2004, 5:29am

That's an interesting idea. Did the author model it out and prove it with science? I am curious how such a planetary system would work. I think it would, I am just curious how.

Title: Re: why ocean doesn't dry up?
Post by BNC on Oct 8^th, 2004, 5:56am

on 10/08/04 at 05:29:22, John_Gaughan wrote:

Did the author model it out and prove it with science?

Not familiar with the story, but since it's Robert L Forward we're talking about, I'm sure he did.

Title: Re: why ocean doesn't dry up?
Post by TimK on Oct 8^th, 2004, 7:39am

Here's a different way of looking at it - what would happen if the water did move 'sideways' because the gravitational force and surface are not perpendicular? The problem seems to imply that this would result in perpetual motion :) Instead, the water would move sideways until the sum of all the non-gravitational forces on it were balanced, where it would reach equilibrium.

Title: Re: why ocean doesn't dry up?
Post by John_Gaughan on Oct 8^th, 2004, 8:36am

If I understand you correctly, this is sort of how Earth really works. Water moves horizontally around the surface perpetually, but it is a globe (no edges) and all the water moves. The net result is zero.