The passive inertia is caused by the passive motion of time-space. The active inertia is caused by the active motion of time-space in spherical time-space. The passive inertia is determined by mass. But, the active inertia is decided by the curvature of time-space and speed of time-space motion. For example, the variable of the reaction in the Newton's law is determined by the active inertia. Reference: THING AND ITS LAW ( ISBN 1-58939-525-5), chapter 5, formula 5.1, published by Virtualbookworm.com publishing Inc.