Earth's unimpeded rotation for billions of years My nephew (7th grade) has stumped me with this question.

If I spin a top, it eventually stops, because of friction, air
resistance etc.  How come the Earth has been spinning non-stop for 4.5
Billion years?  Is there no impeding force (e.g. air resistance), however weak, that could
have stopped the Earth from rotating by now?

 A: If you are asking about frictional forces that would slow the rotation of earth around the sun, then since there is no air in space (space is almost a vacuum), then there is no friction so the earth cannot be slowed down as such. For the same reason, its rate of rotation on its own axis will also not be hindered.
Also, the rotation of earth cannot be slowed down by frictional forces due to the air in the atmosphere since the atmosphere moves along with the rotation of the earth. That is, there is no relative motion between the atmosphere as a whole and the earth. Therefore there is no net external torque on the earth due to the atmosphere, meaning it will not slow down.
If it were the case that the atmosphere rotated in a direction opposite to that of the earth (and wind speeds will be huge), then maybe this might slow the earth (though this will probably be a very small effect as the angular momentum of the earth is huge).
But as has happened in the evolution and formation of earth, frictional drag would eventually slow this down and reverse the direction, to the point where it will once again be in zero relative motion to earth’s surface.
There is also a process known as tidal locking which states that (as per Wikipedia)
The effect arises between two bodies when their gravitational interaction slows a body's rotation until it becomes tidally locked. Over many millions of years, the interaction forces changes to their orbits and rotation rates as a result of energy exchange and heat dissipation.
Click here for more. This process will slow the rotation of earth by a few milliseconds per century. The earth is now tidally locked with the moon (which is why the time it takes the moon to do a full revolution around earth is the same as its time to rotate once on its axis and we constantly see the same side of the moon).
