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Can planet rotation slow down?

  1. Aug 29, 2004 #1
    I going to get straight to the point here. During the "Great Flood" the earth was submerged with water. I cannot calculate how much water that would have to be, or how much that water weighed, but I can guess that all that water did weigh quite a bit. Now with the added weight on the earth would the rotation slow down? I also think that the rotation would not speed back up after the water recessed.
     
  2. jcsd
  3. Aug 29, 2004 #2
    Now, you have to think about this one a little bit... I'm sure you're familiar with the conservation of energy. Well, this is directly proportional to that law. It is impossible for, "all of a sudden" tons of pounds of rain just appearing. That rain had to come from somewhere on Earth, whether it be from the atmosphere or wherever. So, the Earth weighed the same now as it did when that event occured.

    Paden Roder
     
  4. Aug 29, 2004 #3
    Yea

    Good point.
     
  5. Aug 30, 2004 #4

    mee

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    But I do believe it is thought that near the beginning of the earths existence, 14 hour days were had. Nothing to do with a flood.
     
  6. Aug 30, 2004 #5

    russ_watters

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    Staff: Mentor

    The tidal forces between the earth and moon are slowing the earth's rotation.
     
  7. Aug 30, 2004 #6

    Tide

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    Tidal forces on a planet will tend to slow its rotation rate over time.

    Also, a planet's moment of inertia can change if water (mass) is exchanged between polar ice caps and either oceans or the atmosphere.
     
  8. Aug 30, 2004 #7

    Nereid

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    Welcome to Physics Forums AdamS!

    Would you care to say more about this hypothetical 'Great Flood'? For example, you say that during this, 'the earth was submerged with water' - do you mean that the mean sea-level was >8000 metres greater than it is today (so all but a few peaks in the Himalayas were submerged)? If so, where did all that water come from? Where did it go after the flood? When did this flood happen?
     
  9. Aug 30, 2004 #8

    arildno

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    AdamS:
    You're not talking of that Noah-thingy are you ?
     
  10. Aug 30, 2004 #9

    LURCH

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    Let's not make this a discussion of our various religious convictions, folks. If you find a distasteful to view the situation as a literal event, then consider it an hypothetical situation, and hypothesize.

    Adam, yes a planet's rotational period can, and necessarily would change if it's overall mass were to suddenly change or, as Tide pointed out, if that mass or some large portion of that mass were suddenly redistributed from the polls (near the axis of rotation) toward the equator.
     
  11. Aug 31, 2004 #10
    the planet such as the Earth has been slowing down every day. it will stop rotation gradually. the reason is because the air makes a friction with the Earth.
     
  12. Aug 31, 2004 #11

    Nereid

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    There's no doubt about the slowing down! Can you explain a little more about the 'air mak[ing] friction with the Earth' please? AFAIK, the slowing down has to do with exchange of angular momemtum between the Moon (in its orbit) and the Earth (rotating about its axis). If there were no Moon, perhaps the Sun would play a role in slowing down the Earth? Then there's the angular momentum of the liquid core; although it's much closer to the centre of the Earth than the atmosphere, it's also far, far more massive!
     
  13. Aug 31, 2004 #12
    Both the sun and moon make tides on Earth

    Yes. Both the sun and moon make tides.
     
  14. Aug 31, 2004 #13

    Nereid

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    Indeed.

    However, whether the solar tides alone would result in a reduction in the Earth's rotation rate isn't so obvious - after all Mars has a rotation rate of approx 24 hours, and a significantly non-spherical mass distribution, and no liquid core (the two tiny moons - Deimos and Phobos - will have an effect on Mars' rotation rate, but likely quite trivial compared to any solar tides). I'm sure Andre can point us to detailed simulations - as have been done for Venus - that might give us a clue.
     
  15. Aug 31, 2004 #14

    LURCH

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    Yes, the Sun would slow the Earth's rotation to one revolution per year, given enough time, but it would take very long. The same tidal forces that have locked the Moon to the Earth and are in the process of locking the Earth to the Moon would eventually lock the Earth to the Sun, but I've heard it would take longer than the Sun will last for this process to be completed (due to the weekness of the Sun's tidal effect on the Earth.)

    In fact, given enough time, the planets would tidally lock the Sun. Not sure exactly how this would play out, since the planets don't all have the same orbital period, but I suspect the final rotational rate of the Sun would be some logorythmic function of the palnet's orbits, with Jupiter having the lion's share of the influence. But in a single-planet system, the star would have to continually work toward a rotational period identicle to the planet's orbital period.
     
  16. Aug 31, 2004 #15
    Yesterday I wondered why nobody seemed to be interested in terrestrial science anymore. No entries in three-four days. But today so many new treads in my absence. I guess I'm the thread killer here.

    Many interesting suggestions here, pretty well covering the scholar views about tides regression of the moon, etc. Here another interesting link - with sub pages. However, Nereid hinted that there may be more to it than tides alone.

    I propose that the precession of the equinoxes may be the main responsible vector for the Earth slowing down. After all, angular momentum is a vector with a direction. The interaction of Sun Moon gravity with the equatorial bulge causes this vector to point in a totally different direction (46 degrees) after half a period of 13,000 years. The forces only act on the equatorial bulge and hence on the lithosphere-mantle but not on the liquid outer core. This requires transfer of forces and changing speed of masses in the core-mantle boundary region, causing friction, slowing down the spinning and heating up the interior of the planet.

    About Mars I'm sorry but I haven't looked into that yet. The same forces act on Mars but in different strenghts. There have been hints that Mars may have a liquid inner core but only by virtue of the existence of an equatorial bulge. That evidence seems a bit light to me.

    On the other hand, looking at the possible processes that makes planet spin, like contraction of a dust cloud to a solid mass whilst maintaining angular momentum, then the initial spinning rate of Mars should have been considerable less than the Earth and it's slowing down would be orders of magnitudes less than Earth. It's present period of 24,15 hrs may just be another of the many coincidences.
     
  17. Sep 1, 2004 #16
    Changing states

    I think that when it rains the states changes from air to liquid right? If so then wouldn't the weight/mass change/increase?
     
  18. Sep 1, 2004 #17
    Not really. Air is a mixture of gasses, mostly Nitrogen and Oxygen and some traces of other gasses. Among those is water vapor that evaporates from the oceans. The amount of water vapor is dependent on the temperature of the air. If the air cools, water vapors comes out (condenses) and rains down. There is absolutely no change in weight/mass.
     
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