Expert: Courtney Seligman - 12/20/2008

Question
Why does centrifugal force can keeps earth move and rotate on its axis?

Answer
Centrifugal force has nothing to do with how the Earth moves in space. Its orbital motion is due to its inertia, which tends to make it move in whatever direction it is already moving, and to the gravitational attraction of the Sun, which pulls the Earth away from a straight-line path, and into the elliptical path which we call its orbit.

Centrifugal force also has little to do with how the Earth rotates, at least in the sense I believe you mean. The Earth's rotational velocity should make every part of it move along a straight line in space (again, due to its inertia), but the Earth's gravity holds everything together, so instead of flying apart (which is perhaps where your concept of centrifugal force comes in), everything just keeps going round and round.

There is a small deviation, in the case of the rotation, between the effects of gravity as we perceive them, and the actual force of gravity between the various parts of the Earth. This deviation is that portion of the force of gravity which is required to overcome the tendency of the parts of the Earth to move in straight lines (or, in terms of your question, to overcome the centrifugal force). It varies from zero at the Poles, which are not going anywhere as a result of the rotation, to about 1/3 of 1 percent of the force of gravity near the Equator, where the rotational motion is more than a thousand miles per hour. This deviation is sometimes referred to as a Coriolis force. For a discussion of this force and its effects, you might refer to a page on my website, at http://cseligman.com/text/planets/coriolis.htm

Courtney Seligman