Complex sentence: He also solved the problem posed by Mersenne of how to calculate the period of a pendulum made of an arbitrarily shaped swinging rigid body, discovering the center of oscillation and its reciprocal relationship with the pivot point.
Split into simpler sentences: He also solved a problem posed by Mersenne: how to calculate the period of a pendulum made of an arbitrarily shaped swinging rigid body. This involved discovering the center of oscillation and its reciprocal relationship with the pivot point.