Inertia is still a mystery to physics today. The book is presented hypothesis that offers explanations in addition to the nature of inertia (centrifugal) force an explanation of equal gravitational mass and inertial mass. Based on the assumptions of the hypothesis, the orbital radii of the moons of the gas planets are analyzed. The results of the analysis showed that the orbital radii are not random. Confirmation of the correctness of the hypothesis will be a breakthrough in understanding the essence of body matter.