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We all know the earth is rotating around its own axis. From a physics energy point of view, some thing has to provide energy for the rotation. What provides the energy to rotate?

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    $\begingroup$ No energy is required for a body in space to continue rotating after it has initially begun rotating. With no friction, it can rotate forever. $\endgroup$
    – S. McGrew
    Sep 24, 2020 at 21:05
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    $\begingroup$ Downvoted because it should be obvious that the Earth doesn't need energy to keep on rotating, since it is located in a vacuum and so there's very little friction. $\endgroup$
    – jamesqf
    Sep 25, 2020 at 5:35
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    $\begingroup$ @jamesqf: I don't think it's obvious. In fact OP's misunderstanding was believed for 1000's of years. It wasn't until Galileo in the 1600's that inertia was understood. $\endgroup$ Sep 25, 2020 at 5:43
  • $\begingroup$ Possible duplicates: physics.stackexchange.com/q/12140/2451 , physics.stackexchange.com/q/142014/2451 and links therein. $\endgroup$
    – Qmechanic
    Sep 25, 2020 at 8:58
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    $\begingroup$ I object closing the question because it is different from those who are mentioned as duplicates. "What provides energy" means a continuous provision of energy. Initiation and origin assumes the energy was provided initially. $\endgroup$ Sep 26, 2020 at 13:28

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The original energy that caused the rotation of the Earth when it was formed came from the gravitational collapse of the cloud of dust and gas that formed the solar system. Maintaining rotation at constant speed in a vacuum does not require any further input of energy. Conservation of angular momentum keeps the Earth rotating.

In fact, over long time scales (millions of years) the Earth’s rotation does slow down gradually due to energy lost in friction and tidal forces.

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  • $\begingroup$ can they also speed the rotation up? $\endgroup$ Sep 25, 2020 at 8:02
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    $\begingroup$ @BЈовић: Yes. If we started from a situation where the earth was not rotating and the moon was in its present orbit, then tidal forces would cause the earth to start rotating, until its rotation was synchronous with the moon's orbit. (This would take a very long time!) $\endgroup$
    – TonyK
    Sep 25, 2020 at 9:34
  • $\begingroup$ @BЈовић An asteroid or comet impact could also impart angular momentum (positive or negative) to the Earth. Something that came in tangentially, near the equator would have the greatest effect. If it came straight "down" (if its direction vector passed through the centre of the Earth), it would have no effect. A nice exercise in vector algebra... $\endgroup$ Sep 25, 2020 at 10:46
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    $\begingroup$ @DescheleSchilder ... not really, some planets rotate in a weirder way; please refer to en.wikipedia.org/wiki/Axial_tilt $\endgroup$
    – dominecf
    Sep 25, 2020 at 12:08
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    $\begingroup$ @dominecf: lunar tides are more than twice as strong as solar tides. (I didn't need to do any "serious research" to find this!) $\endgroup$
    – TonyK
    Sep 25, 2020 at 12:15
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If there is no friction a rotation needs only initial energy. But earth has some friction caused by the tides, so the rotation gets slower with the time. But its only about 1ms less per day in 50 years.

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  • $\begingroup$ This is no answer to the question in the question body. -1 $\endgroup$ Sep 25, 2020 at 12:26
  • $\begingroup$ How big is this torque, based on actual observation of torque? (As opposed to calculating it from observed angular velocity changes.) $\endgroup$ Sep 26, 2020 at 13:30
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Angular momentum is a conserved property in physics. Once something rotates, one needs some effort to make it stop.

In our usual environment, friction acts to stop the rotation of different object, no matter if we want it or not. That's why we are used to the need of energy to sustain rotation (or any other movement).

Earth, the other hand, does rotate (as far as we know) from before it was formed. The gas and dust that formed the Earth rotated and nothing stopped them as they coalesced into a planet.

Earth doesn't experience much of a friction or torque, compared to its own moment of inertia. There is near-perfect vacuum around. The major force slowing down the Earth is the tidal friction. The effect of tides is very small on human timescale, but measurable with modern tools. There is also geological evidence of shorter days (=faster rotation) in the distant past.

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  • $\begingroup$ Apparently dinosaurs were living in a 23.5 hours day long, and 372 days long year. $\endgroup$ Sep 25, 2020 at 9:13
  • $\begingroup$ @thermomagneticcondensedboson Can one calculate from that fact (65 million years ago), assuming the conditions stay the same, how long a day will last about 5 billion years from now( before the Sun blows the Earth apart)? Of course. I'm just curious what the answer is. $\endgroup$ Sep 25, 2020 at 14:10
  • $\begingroup$ @DescheleSchilder keep in mind that the Earth is slowed down not only by lunar tides, but also by solar ones. The orbit of the Earth around the Sun gradually widens and the year gets longer. How much exactly 5 bilion years are? Also, in ~100 milion years there will not be water on Earth, so the most important tides won't happen. $\endgroup$
    – fraxinus
    Sep 25, 2020 at 15:50
  • $\begingroup$ @fraxinus So in 5 billion years the Earth will move further from the Sun and rotate slower (though not that much because of the lack of water) And of course we must assume that no catastrophic event happens. What if all planets are aligned on the same side of the Sun? $\endgroup$ Sep 25, 2020 at 16:22
  • $\begingroup$ @DescheleSchilder we are going pretty much offtopic, but see here phys.org/news/2016-10-link-solar-tidal-effects-venus.html . Our lovely Sun also experiences tides and they have pretty much visible indirect effects. $\endgroup$
    – fraxinus
    Sep 25, 2020 at 16:57
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Your question is a bit unclear, so I'll give two answers:

  1. Nothing needs to provide energy for the earth to keep spinning. In fact, the rotation is slowly slowing down due to angular momentum being lost to the moon, losing energy to the moon's orbit (it's pushed away from earth), and the tides (lots of friction).

  2. However, the energy does have to have come from somewhere. And it did. Notice that I said that the moon is getting farther away from the earth, slowly. Where do we get to when we rewind time? The moon must have formed close to an insanely fast-spinning earth. Where did this fast-spinning come from?

    Well, the current theory on this is, that both earth and moon formed in a giant collision between the proto-earth and another planetoid called Theia (https://en.wikipedia.org/wiki/Theia_(planet) ). This impact was not head-on, Theia hit the earth with quite an offset. It is this offset that supplied the angular momentum to give the cloud of debris a fast twist. This is the energy and angular momentum that earth is still drawing upon when it accelerates the moon along its orbit and makes the tides rise and fall throughout the planet.

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  • $\begingroup$ Somewhat unclear. After the collision with Theia, the Moon was formed out of proto-Earth. Also, you write: "The moon must have formed close to an insanely fast spinning earth." Why Earth was insanely fast spinning? Did proto-Earth spun insanely fast? If the spinning induced was by the collision with Theia, was proto-Earth already spinning? In which direction, relative to the direction of the rotation did the collision occur? At an angle of 23 degrees? Was this collision the reason for the Moon always showing to us the same side? Maybe proto-Earth had already enough spinning energy. $\endgroup$ Sep 25, 2020 at 17:54
  • $\begingroup$ @DescheleSchilder Well, since angular momentum is conserved, and since the moon is constantly draining angular momentum, from earths rotation, Earth must have been spinning pretty fast when the moon formed in low earth orbit. It took a lot of angular momentum and energy to place the moon where it is today, and you can easily calculate back just how much angular momentum. How the proto-earth has been spinning before the impact, we don't have a clue. The impact itself delivered an amount of angular momentum that easily dwarved any preexisting spins of proto-Earth and Theia. $\endgroup$ Sep 25, 2020 at 21:18

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