The Eotvos experiment tests the equivalence of gravitational and inertial mass. Some descriptions state that the experiment tests the equivalence between gravitational and centrifugal force, while others state that it is between gravitational and centripetal force. There is a significant difference between the two: Centrifugal force is a fictitious force, like inertial force, and its cancellation with gravitational force would require that the two masses in the torsion balance must be effectively in gravitational freefall. Centripetal force is a real force, and its balancing would require that the two masses not be in freefall and experience the gravitational force. So does the experiment involve centrifugal or centripetal force?