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.
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
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)