Centrifugal is imaginary in a non-inertial reference frame (as the case is here), but is a "real" force otherwise. There was a discussion on this exact subject

Centrifugal is imaginary in a non-inertial reference frame (as the case is here), but is a "real" force otherwise. There was a discussion on this exact subject recently on Reddit (/r/askscience) which might be interesting: http://www.reddit.com/r/askscience/comments/m2b1t/how_fast_could_a_rock_planet_spin_before/c2xioo7 http://www.reddit.com/r/askscience/comments/m2b1t/how_fast_c... and the entry on Wikipedia: http://en.wikipedia.org/wiki/Centrifugal_force http://en.wikipedia.org/wiki/Centrifugal_force