Quick Answer
If Earth rotated twice as fast while keeping its mass and shape initially unchanged, the day would be about 12 hours long. Centrifugal effects would be stronger, especially near the equator, slightly reducing effective weight there and increasing Earth’s equatorial bulge over time. Atmospheric and oceanic circulation would also reorganize.
Earth’s current rotation sets the basic length of the solar day and influences atmospheric circulation through the Coriolis effect.
A 12-hour day
A simple doubling of the rotation rate would roughly halve the sidereal rotation period. Sunrise and sunset would arrive twice as often, although the exact solar day depends on Earth’s orbit as well.
Gravity feels slightly weaker at the equator
Faster rotation increases the outward centrifugal acceleration that slightly opposes gravity. The effect is still small compared with gravity itself, but it would be strongest at the equator.
Weather changes
The Coriolis effect depends on rotation rate. Doubling it would alter the size and behavior of atmospheric circulation patterns, storms and ocean currents.
The planet would adjust its shape
A rapidly rotating fluid-like planet tends to become more oblate. Over long timescales, Earth’s equatorial region would respond to the stronger rotational effect.
Established scienceRotation does not create gravity, but it changes effective weight by adding a small outward inertial effect in the rotating frame.
The bottom line
Twice the rotation rate would make days shorter and reorganize climate and circulation. It would be noticeable everywhere but would not make people float away from Earth.
A note on our approach: Every article on WhatIfLab separates what current science establishes from what remains genuinely speculative. Where we cite a figure or finding, it reflects published, mainstream research at the time of writing — not a prediction dressed up as fact.