Do we rotate along with it because of it's gravitational pull? Or is it friction?
Why do you accelerate together with a moving bus while standing in it?
Well, we are already moving with the Earth, friction allows us to accelerate in any direction without sliding and keeps us in place, and gravity keeps us pulled down to the surface.
I prefer the second answer: we have always been moving with earth, so neither friction nor gravity are required to keep us moving with the earth.
The OP asks about rotation along with the Earth. You can go to the North Pole, rotate slowly againts the Earth's spin, and you are not rotating along with the Earth anymore. So you cannot generally say that everyone has been always rotating with the Earth.
We need both.
If we had neither, we would be attracted towards the sun which, presumably, would still have gravity.
If we only had gravity, we would watch the slippery earth moving underneath us, but we would remain stationary.
If we only had friction, we would be projected tangentially away from the earth into space.
Ok, almost everyone -- 99.99999% of the population (estimate).
Stationary wrt what?
With respect to the center of the earth.
Instead of being rotated, we would just be dragged towards the earth by gravity, but we would be unable to penetrate the earth, so we would just cling to the outside, but not rotate with it.
It would be a fairly terrifying existence as we would be unable to even drill holes in the earth to get a grip on it because it was moving so fast.
I saw a calculation that gave the speed of the earth's rotation, at the equator, of 1070 miles per hour or 0.297 miles per second.
Again, all that assumes we are placed on earth stationary wrt its rotation.
The reason I consider this caveat important is that in our daily lives, friction does not play a role in keeping us rotating with the earth. Most people asking such questions seem to think that if friction suddenly disappeared, we would start being left behind earth's rotation.
....unless we move significantly north or south, as per A.T.'s point.
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