Complex sentence: The concept of reactive centrifugal force is seldom used in modern physics and mechanics, having been largely supplanted by the concept of centrifugal force as a fictitious force in a rotating reference frame; see History of conceptions of centrifugal and centripetal forces.
Split into simpler sentences: The concept of reactive centrifugal force is seldom used in modern physics and mechanics. The adjective "centrifugal" is most often used in relation to a different phenomenon: fictitious force in a rotating reference frame; see History of conceptions of centrifugal and centripetal forces.