If the planet stopped suddenly, everything on the surface would be destroyed, as the atmosphere, oceans and anything not nailed down kept spinning. Even braking to a halt over a minute would mean everything experienced a sideways deceleration of three-quarters of Earth’s gravity, so ‘down’ would feel like it was at an angle of 38° from the vertical. That’s enough to knock over most buildings.
2. Changing continents
If it slowed down over several years, it would still be a disaster. Without centrifugal force, the oceans would move towards the poles, dropping ocean depth by 8km around the equator. Since this is less than the depth of the ocean there, Earth’s water would be divided into two huge polar oceans separated by a belt of land in the middle. Everything north of Spain would be underwater, as well as all of Antarctica.
3. Bake and freeze
Once Earth doesn’t spin on its own axis, a day lasts as long as a year. Everywhere receives six months of daylight, gradually heating up the planet to well over 100°C. The huge central continent would get the hottest and any remaining lakes and rivers would boil away and be blown to the poles by fierce winds. Even primitive life would only be possible along a narrow strip at the coast.