Gravity: Earth vs. Venus

Discussion in 'Astronomy, Exobiology, & Cosmology' started by Jocariah, Oct 24, 2005.

  1. Jocariah Registered Senior Member

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    Venus has nearly the same mass, orbital velocity and is nearly the same distance to the sun as is Earth, but rotates at 1/250th the rotational speed of Earth.

    Venus also shares gravity nearly equal to Earth’s, although it rotates at just 1/250th the speed of Earth (as a sidebar, its day is actually longer than its year).

    How can this be? How can its gravity be nearly equal to that of earth’s, while its rotation speed is only 1/250th that of Earth?

    Thank you
     
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  3. Light Registered Senior Member

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    The speed of rotation has no real effect on the gravity (perhaps very slightly at the equator) it's all a matter of mass.
     
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  5. Lucas Registered Senior Member

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    One of the explanations put forward is that in the early stages of the Solar System, Venus was struck by a planetesimal coming from a direction opposite its rotation, and the impact slowed its rotation and tilted greatly its axis
     
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  7. Pete It's not rocket surgery Registered Senior Member

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    For the same reason that gravity on Earth at the North and South poles is nearly equal to that at the Equator: Earth's rotation doesn't make much difference.

    You can work it out. The difference in apparent gravitational acceleration at the equator because of Earth's rotation is the centripetal acceleration of objects on the Equator.

    The equation for centripetal acceleration is:

    a = r.ω²

    Where:
    a = centripetal acceleration ( m/s² )
    r = radius of rotation ( m )
    ω = speed of rotation ( radian/s )

    For Earth at the equator:
    r = 6380000m
    ω = 1rev/day = 2pi radian / 86400s = 0.0000727 radian/s

    a = 0.0377 m/s²
    This is all the difference that Earth's rotation makes - less than half a percent of the standard 9.8m/s².
     
  8. Pete It's not rocket surgery Registered Senior Member

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  9. DwayneD.L.Rabon Registered Senior Member

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    Last edited: Oct 28, 2005
  10. Jocariah Registered Senior Member

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    First let me thank you for your kindness in providing all of the above information, which I read with great interest.

    Am I to understand, then, from the above, that mass, in and of itself, has no inherent gravity, except when it is experiencing a minimum, or threshold rotation speed, and that above this threshold rotational speed, for any given mass, the gravity produced does not increase in any real way (i.e., the greater the rotational speed – the greater the apparent gravity produced, does NOT apply)?

    Spin a mass approximately the size of Earth at 1/250th the speed of Earth (e.g., the speed of Venus), or indeed 250 times faster (e.g., the speed of Earth) - the gravity will still be the same .... and if mass, in and of itself, has no gravity, there must be a point at which a threshold rotational speed is achieved in order for gravity to be present for a rotating mass.

    This all seems counter-intuitive to me - as here on earth, the faster you spin a centrifuge, the greater the perceived G-force by the occupant. (EDIT: ..or is this centrifuge example really a matter of acceleration vs. gravity?...)

    Thank you


    ...
     
    Last edited: Oct 25, 2005
  11. DwayneD.L.Rabon Registered Senior Member

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  12. DwayneD.L.Rabon Registered Senior Member

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  13. Jocariah Registered Senior Member

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    The fact that Venus rotates at 1/250th the speed of Earth and yet has the same gravity seems counter-intuitive to me. One would think that the rotational speed of a body would have a direct (i.e., proportional) bearing the gravity it produces.
     
  14. Lucas Registered Senior Member

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    Uh? Hydrogen core? The core of Venus is made of iron, and Venus has never had an hydrogen core. Of course, the rest of your post seems quite wrong
     
  15. Pete It's not rocket surgery Registered Senior Member

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    No, I don't know how you got that.

    If you idealise Earth to be a sphere of uniform density, then the acceleration due to gravity on the surface of Earth is determined by three things:

    M = The mass of Earth.
    R = The distance from the center.
    G = the gravitational constant

    g = G.M/R²

    This works out to be about 9.8 m/s².
    The exact number varies slightly from place to place, because the Earth is not uniformly dense, and it is not a perfect sphere.

    OK? SO this is the gravity that's there because of Earth's mass.
    It would be there if Earth was not rotating at all. 9.8m/s²
    It's much the same at the Poles as it is at the Equator (a bit more, actually, because the Poles are closer to Earth's center).

    The reduction in apparent gravity at the Equator is directly proportional to the rotational speed of the body. For Venus it's effectively zero, but it is still very small for Earth. For both planets, the dominating factors are the mass of the planet and the size of the planet.

    More detail:
    Because Earth is rotating, the surface at the Equator has some real acceleration toward the centre of the Earth. This real acceleration makes the apparent acceleration due to gravity seem to be a bit smaller than what it really is. Or if you like, things are "flung out" a little bit by Earth's rotation, reducing gravity.

    I showed you earlier how to work out this reduction, and it turns out to be 0.038m/s² at the Equator - ie it's such a small difference that you don't notice it. What's more, the difference is even less away from the equator, reducing to zero at the poles.
     
  16. DwayneD.L.Rabon Registered Senior Member

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  17. Light Registered Senior Member

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    Oh, good grief! The local boy genius/idiot surfaces again!

    Rabon, why don't you go back in the other room and play with your Leggos again? Leave nice people alone who are actually trying to learn something. They certainly don't need to have to read your absurd ideas that make no sense at all and actually are completely worthless.

    (Hydrogen core indeed - just how stupid can one guy be?!?)
     
  18. Lucas Registered Senior Member

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    But that is just your pet theory, and you should advertize it clearly

    The truth is that is difficult to find something correct, but just as an example


    No chance in hell! The tidal braking exerted by the Moon on Earth is indeed slowing down the rotation of our planet, but the rate of slowing is very small, in fact tidal braking is lengthening the day (earth rotation around its axis) only by 0.002 seconds every century
     
  19. Tortise Registered Senior Member

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    No chance in hell! The tidal braking exerted by the Moon on Earth is indeed slowing down the rotation of our planet, but the rate of slowing is very small, in fact tidal braking is lengthening the day (earth rotation around its axis) only by 0.002 seconds every century

    This is true. The moon will eventually dictate the rotation intervals of the Earth. By that time the moon will be in a larger orbit - will be further away from the Earth. The days will be longer then 30 of our days today.
    Also someone asked why Venus' days were longer then it's years. I think that's because it's in some sort of resonance with the Earth, because I think the time interval coincides closely with the interval when the proximity to the Earth is closest.

    The length of the day is increasing by 0.0015 seconds every century, of which about 0.0007 seconds per century has to do with the tidal breaking of the Moon. As a result of this, the Moon's orbit must also increase so that the Moon is slowly getting farther and farther from the Earth by a few centimeters per year or so ( my estimate!). As this process continues, it is predicted that in billions of years the lunar month will increase to about 47 days from its current 27.3 days. But by that time, the Sun itself will have begun to evolve into a red giant, which will upset the Earth-Moon system somewhat; especially if they are both engulfed by the Sun's expanding atmosphere!
    -Goddard Space Flight Center
     
    Last edited: Oct 27, 2005
  20. DwayneD.L.Rabon Registered Senior Member

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  21. Tortise Registered Senior Member

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    The almost complete lack of friction (of the Earth) is what diferentiates the spinning top from the spin of the Earth as an analogy.
     
  22. Light Registered Senior Member

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    Rabon, let me make something perfectly clear. I would not ridicule you at every turn if you were to actually say something sensible. But 90% or more of everything you say is utter nonsense.

    You claim all this great scientific knowledge and brainpower, yet you do not know many of the very basics of science. Your wild and silly ideas go against practically every established scientific fact.

    You picture yourself as a great visionary with insights that ordinary people and scientists cannot even grasp. But the real truth is you haven't one single fact to support what you say. Not one.

    And if your IQ was even 20% of what you claim, you'd be able to spell correctly and form coherent sentences - which you absolutely cannot do. Obviously you aren't capable of even the simplest things.

    Your so-called education is sorely lacking and you prove it every time you make a post. In truth, you are nothing more than a home-schooled kid who has no idea at all of how the Earth and Universe are put together.

    The only people who will ever buy into your garbage are the poor individuals much like yourself that know almost nothing.
     
  23. Lucas Registered Senior Member

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    Yes, the moon is receding from Earth, but many years ago, when it was closer to Earth, an Earth day was only 4-5 hours long. But the moon has been slowing down Earth until the actual 24 hours day, and will keep slowing down it until Earth and Moon become tidally locked to one another (as for example Charon and Pluto), and then Earth won't slow down no more, its day being 47 times the length of its present day. So the idea expressed by Rabon that Earth will stop rotating is not correct
     

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