How fast would the Earth have to rotate for centrifugal force to overcome gravity?

The answer is the equator would have to travel at about 15,000 miles per second (versus 1,030 miles per HOUR now), giving a day of about one-and-1/2 seconds.

At that speed the effect of heights would be exaggerated. Airplanes would whiz off into space, while only a few feet underground things would be relatively stable.