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Three questions arose to me:

The first is what is responsible for the earth spinning? Is this just newton's first law in action - having been formed out of the material in the cluster of clouds after the big bang the world just remained in circular movement?

The second - are there any known planets that do not rotate on their axis?

The third - what is responsible for the objects on earth to spin with the earth to that rotation is left unnoticed? Is it the gravitational force of the earth that, as the objectד on earth have radial velocity the gravitation pull gives the objects a circular motion, which incidentally is the same as the the earths rotation?

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  • $\begingroup$ The Moon is tidally locked to the earth, which is why you can see the man in the moon all the time. It does rotate, but only as it follows it's rotation around the Earth. $\endgroup$ Apr 13, 2022 at 19:39
  • $\begingroup$ @foolishmuse Right, hence - the dark side of the moon... But I was asking about a planet, and not a moon. $\endgroup$
    – Cantor
    Apr 13, 2022 at 19:41
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    $\begingroup$ Does this answer your question? How do the Planets and Sun get their initial spin? $\endgroup$
    – g s
    Apr 14, 2022 at 1:29
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    $\begingroup$ I have voted to close this question because it is a duplicate. 1: has answers here: physics.stackexchange.com/q/23104 3: has answers here: physics.stackexchange.com/q/299723 $\endgroup$
    – g s
    Apr 14, 2022 at 1:35
  • $\begingroup$ To 2: if you mean tidal locking - one rotation per orbit, like the moon - Kepler10b is known to be tidally locked to its star. Tidal locking of exoplanets is probably common. If Pluto currently counts as a planet, Pluto and its moon Charon are tidally locked. If you mean literally not rotating, near-zero angular momentum with reference to its axis, no. Any planet that started with near-zero rotation would begin to rotate because of tidal drag. $\endgroup$
    – g s
    Apr 14, 2022 at 1:42

1 Answer 1

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  1. Conservation of angular momentum. If there is no torque on the Earth then it will keep rotating, given its initial condition, when the solar system was formed.

  2. Well if a planet does not rotate, how do you define its axis?

  3. For a body to undergo circular motion, as we humans do on top of Earth's surface, it needs a centripetal force. A very tiny part of Earth's gravity provides the necessary centripetal force, the remaining large portion of gravity must be cancelled by the normal reaction of the Earth's hard surface. In other words, our ``weight" is the gravitational pull minus the centripetal force. Note that if the gravitational force would equal the centripetal force we would be afloat. In other words, we would be orbiting the Earth, precisely as the moon does around the Earth.

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