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I was reading two sites before I posted the question here. One site says something like that the collisions in Universe eventually set the planets rotating, another site says that our planet is formed from some collapsing Nebula or whatsoever and Nebula was rotating when it was about to die and that set the Earth rotating at first place.

The question should be probably better specified. Is Earth rotating because of some continuous force, or because at first place there was some one-time force which set it rotating and the effect lasts until now?

As for the first case, which force is it? As for the second case, that should break our physical laws, because there would be one force, applied a very long time ago which is still in effect and not about to slow down and with a very very long lasting effect. Because thousands or millions of years for rotation is a very long time, isn't it. Or is it slowing down?

BTW: the infinite movement around orbit is bit mysterious too, because you can say that's because of gravity, but gravity itself attracts the things only to one direction, but orbit movement is like parallel with the gravity emitter, and again it is perpetual.

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  • $\begingroup$ This existing question might answer some of your questions (though not all) $\endgroup$
    – Andy
    Sep 21, 2016 at 10:12
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    $\begingroup$ You do not have to apply a force for something to continue to move. It is called inertia. Things continue to move until you apply a force to change the movement. $\endgroup$
    – SE - stop firing the good guys
    Sep 21, 2016 at 10:33
  • $\begingroup$ Old joke: "love makes the world go 'round ... with a little help from intrinsic angular momentum". But the question you ask is valid. Under constant force, every part of the Earth should travel in a straight line, not rotate. Rotation occurs because the Earth's outer shell is solid, which means it's held together by the electrical force. Technically, the Earth's rotation does place a stress/strain on the Earth's crust, but it's not strong enough to tear the Earth apart. $\endgroup$
    – user21
    Sep 22, 2016 at 0:49
  • $\begingroup$ it's not infinitely spinning. It's slowly (extremely slowly) slowing down. the only reason it is (very slowly) slowly down is various types of friction. $\endgroup$
    – Fattie
    Sep 23, 2016 at 16:07
  • $\begingroup$ Yea but inertia eventually slows down. But question is then what is slows down. I guess then a friction, gravity and i thought also the movement itself (i am unsure about this one - but would be logical from view that energy transforms always to something - in this case to rotation - so with every spin it should be consumed also with rotation itself). Anyway, thanks all. $\endgroup$
    – luky
    Sep 28, 2016 at 9:12

1 Answer 1

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As for the second case, that should break our physical laws

That is precisely the wrong statement. The fact that the Earth is still spinning matches our physical laws exactly. The physical law I'm specifically referring to is known as Newton's First Law of Motion, which is loosely paraphrased as:

An object at rest remains at rest unless acted upon by a net, outside force. Conversely, an object in motion remains in motion unless acted upon by a net, outside force.

What this means is that once something starts moving (like the Earth spinning) it will keep moving until something else stops it. The Earth long ago started spinning because of the initial angular momentum of the collapsing cloud which eventually made up the Earth. There is nothing in space to stop the Earth from spinning so it is just going to go on spinning1.

You appear to believe that if you start an object moving by applying some force, it will eventually run down and stop moving of its own accord. This is a common thought, based on everyday experiences. In fact, that was what Aristotle thought (and much of the world up until Isaac Newton's time). But what you're not taking into account is that there's always a force being applied to slow things down. Consider a car rolling along the highway. If you take your foot off the gas pedal, the car starts slowing down. But it isn't because some physical law says it should, it's because a net force is acting on the car to slow it down $-$ that net force being wind resistance on the car. So you see, even common, everyday experiences hold up to Newton's first law.

1Technically there are some small forces acting on the Earth to slow it down but those are nearly negligible and mostly from the Moon. These forces would take billions of years or longer to make the Earth come to a complete stop (if they ever even could).

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    $\begingroup$ Newton's First Law is conservation of momentum. Conservation of angular momentum is more relevant (but closely related). $\endgroup$
    – Keith Thompson
    Sep 21, 2016 at 19:47
  • $\begingroup$ Thank you for a profound answer and also to all participants of the question. So you say that angular movement itself does not consume any energy? I would think that energy is transforming, that means every spin should consume the initial energy a bit, since rotation is transformed energy of the initial force, but according your answer it seems that rotation (and any movement) cost no energy at all. $\endgroup$
    – luky
    Sep 28, 2016 at 9:10
  • $\begingroup$ Second thing, so earth is rotating thx to momentum, that means no continuous force is applied. That's interesting anyway, if you consider it lasts like few millions of years yet ! $\endgroup$
    – luky
    Sep 28, 2016 at 9:15
  • $\begingroup$ btw according the statement "Conversely, an object in motion remains in motion unless acted upon by a net, outside force." the perpetum mobile is perfectly possible if one will find the way how to remove the stopping forces, and Earth spinning millions of years is really some kind of PM too. $\endgroup$
    – luky
    Sep 28, 2016 at 9:30
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    $\begingroup$ @ranjithstar256 Actually in that case, the internal friction of the fidget spinner itself will slow it down. The fidget spinner is a device rotating on a central rod that isn't rotating. There's going to be friction between the two that applies a force to slow the fidget spinner down. A better analogy would be to spin a simple ball in space. However, the comments here are not a good place for back and forth questions or discussion. Feel free to ask new questions in the appropriate way if you're still curious. $\endgroup$
    – zephyr
    Oct 25, 2017 at 14:17

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