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Let's say through some magic or other, the Earth's spin came to a halt (to the point that the same side now always faces the Sun)

How would this process feel? Obviously it would depend on how long it took to do so, so let's say over the course of an hour.

This question is specifically about the effects that occur during the change/slow-down, rather than just the effects of the Earth not rotating. Major flooding as the seas wash up against the shore? Buildings falling down? Trees? That kind of thing... Or would we even notice it? (after all, we don't notice the current spin of the Earth)

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The answer depends on latitude, so let's look at the worst case along the equator. At the equator, the rotation of the earth gives us a linear motion of 1670 kilometers/hour, or 463 m/s. If you reduce this to 0m/s over the course of an hour, that corresponds to a deceleration of 0.129m/s^2.

This is a small number by human terms. A car decelerates at shy of 10m/s^2, nearly 100 times more aggressive than our slowdown will be. This rate is roughly 1/10th the acceleration you feel when an elevator starts to go up.

Things which we tend to think of as responding to accelerations will not be hurt by this kind of change. However, things we tend to think of as either solid or unrelenting may require further investigation.

This means trees wont mind anything, and neither will the buildings. Mountains would generally not be bothered (though some particularly precarious monuments may topple).

The two places I would expect to see noticeable effects is the ground and the ocean. The ground would be put under intense strain by this kind of acceleration, simply because it is a massive mass which has not settled in a direction that deals with that acceleration well. I would expect earthquakes in all of the regions traditionally associated with earthquakes, but I would not expect them to extend into more stable areas (the ground would probably be stable enough to take the beating).

The oceans are interesting because of momentum. To draw an analogy, consider one of these toys http://www.amazon.com/ALPI-Liquid-Wave-Paperweight/dp/B004P93Y78 . The effect on the ocean would be remarkably similar to tilting the paperweight a few degrees. The water rushes towards its new "level." The initial effect would be nothing more than some minimally rising tides. However, in the middle of the ocean, there would be an opportunity for the water to build up an enormous amount of momentum, just like the waves in the toy can build up. This happens because the flowing water is remarkably good at conserving energy as it shifts, so it simply continues to pick up velocity until the land gets in its way.

The worst case scenario? A volume of water hits the land just after the rotation has stopped, without losing any of its initial velocity from spinning with the planet. This would be a 463m/s tidal wave of epic proportions, causing massive damage in its wake. In reality, it won't be perfect. Interaction with the ocean floor would cause some slowing. A tidal wave tend to hit around 200m/s, so if 50% of the energy gets conserved, a tidal wave the size of the ocean will hit our seaboards, all at once.

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There is a TV show that covered that exact scenario (video). A search turns up a lot of commentary on it.

One thing Cort Ammon did not mention is that the air and water would drain away from the equator: the oblateness of the earth would turn into high ground: 13 miles higher than at the poles. That's 70,000 feet, which is rather thin air indeed.

I don't know how long it would take the earth to return to a spherical shape, whether it's thousands or millions of years, but think of the speed at which the Himalayan mountains sag.

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  • $\begingroup$ That is true, I was focused on the hour where the planet was actually slowing, but once it's stopped, you're right. That effect would be rather dramatic indeed! $\endgroup$ – Cort Ammon May 1 '15 at 6:11
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Most things that could stop the Earth's rotation suddenly (e.g. Thea Event https://www.youtube.com/watch?v=Fwl_JBQtH9o ), would vaporize or liquify a significant portion of the planet.

We wouldn't be around to worry about the piddly little stuff like atmosphere and hydrosphere.

Other things that might cause the Earth to stop rotating would stop all the objects on the Earth from rotating too (e.g. it'd keep the oceans and atmosphere in place).

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Depends on how the earth stops. It would take an effect never before observed to stop the earth's rotation.

If the stopping were caused by some sort of obstacle or friction applied to a particular location, that location would suffer massive damage in addition to the effects Cort Ammon described.

But if it were caused by something that affected everything on the planer, like gravity or a sci-fi "tractor beam," then there would be no consequences as Jim2B suggested.

If it were some strange phenomenon that did not affect liquids, the tidal waves Cort described would occur. If it affected only liquids, they would occur in the opposite direction and perhaps be even worse as the water would not slow the land as fast as the land could slow the water.

If air were not affected, there would be increased winds. I think the increases would be modest, but being world-wide, perhaps they would add up to a siinifcant affect on weather.

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