What would happen, if the earth spin was twice the velocity or half the velocity?

Besides the obvious change in the length of a day, a change in the earth's rotation rate would cause changes in centrifugal force and the Coriolis effect. Presently, at the earth's equator, the centrifugal acceleration is about 0.003 g. Doubling the rotational velocity would increase this to 0.014 g, while halving the rotational speed would reduce it to less than 0.001 g.

calctool.org/CALC/phys/newtonian/centrif... The centrifugal force with a 12-hour rotational period would therefore cause a person to weigh just over 1% less at the equator. Centrifugal force would also have an effect on the earth's surface area. A faster rotation rate, with greater centrifugal force, would cause the earth to become more oblate, increasing its surface area.To accommodate the increased surface area, the earth's crust would have to become thinner and/or break apart in places, resulting in the formation of large-scale spreading centers (new divergent plate boundaries) and volcanic activity.

Conversely, slower rotation would result in a more spherical earth, resulting in a decrease in surface area and the formation of folded mountain belts as the tectonic plates are compressed. Changes in the Coriolis force would affect the earth's climate. Atmospheric circulation patterns are in a balance between the Coriolis force and pressure gradients, resulting in winds circulating around areas of high and low pressure as well as zonal features such as the jet stream.

en.wikipedia.org/wiki/Coriolis_effect While I could find no sources citing specific effects, an increase in the Coriolis force from a faster rotation rate would cause winds to move less directly across pressure gradients and increase the circulation around areas of high and low pressure. These areas, including storm systems, would therefore be more persistent, with more intense circulating winds. A decrease in rotation rate would have the opposite effect.

Zonal circulation would also be affected: The outer planets, with relatively rapid rotation, are dominated by east-west bands or belts; while Venus, with very slow rotation, shows a more diffuse pattern of clouds in a diagonal pattern, converging at the equator.

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