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Q & A: Earth stop spinning?

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Most recent answer: 11/13/2015
What would happen if the Earth stopped it’s rotation?
- Mark
Dallas, Texas
Sometimes 'what if' questions are pretty hard to answer, because the 'what if' couldn't happen without something else being drastically different. The Earth couldn't stop spinning without either:
1. having some enormous torque on it, maybe from a collision with another large object
2. The laws of physics (conservation of angular momentum) changing.

If the laws of physics suddenly changed, then we couldn't use them to guess what would happen, so you'd have to ask somebody other than a physicist. If the Earth collided with some very large object, we'd be in deep trouble. Actually, that's not true, we'd simply be dead.

One good question, which I'm not qualified to answer, is whether there's any way the Earth could bump into something big enough to stop the spinning without also being broken up into pieces.

Mike W.

Actually, the Earth's spin is slowing down very gradually due to tidal friction with the moon. As the tides slosh around the Earth (and there is tidal motion of the ground, too!), they exert a frictional force on the spinning Earth. The total angular momentum of the Earth-moon system stays the same, and the moon is gradually picking up angular momentum and energy as the Earth loses it. This process would continue until one lunar orbit around the Earth takes the same time as one revolution of the Earth around its axis, but the Sun will probably blow up first. The opposite effect has already happened -- tidal friction has already locked the moon's rotational period equal to its orbital period.

This whole process wouldn't stop the spin of the Earth, just slow it down, making the days and nights very long. Also, ocean currents and prevailing winds behave the way they do in part due to the Coriolis "force", a fictitious force like centrifugal "force". With a weaker Coriolis force, air wouldn't swirl around low pressure zones as much and the weather would be different. The weather might still be violent as convective currents would do their part to bring the day side of the Earth and the night side of the Earth closer in temperature.


(published on 10/22/2007)

Follow-Up #1: What if Earth stopped spinning?

What would happen if the earth stopped spinning my son has just asked me and I don't know does anyone here know
- Alexander (age 34)

We've basically answered this question before, up-thread here. It's a nice time to get it again because I'm trying to explain to a philosopher what we mean when we ask what causal effects are. You can't say anything about those effects unless you know the rules of the game, i.e. the laws of physics. And you can't say what the effect of the Earth stopping spinning would be unless you know how to fit that event into the rules of the game.  It seems like some big collision would be needed. For practical purposes, I doubt it would matter whether that collision happened to stop the spinning or speed it up. Either way we'd be done for.

Mike W.

(published on 11/13/2015)

Follow-up on this answer.