When the reference frame is a non-inertial frame, the object is in a state of variable speed motion with acceleration when it is not subjected to external force. The external force here is relative to the inertial system, relative to another reference system, not this reference system. "The object is in a state of variable-speed motion with acceleration" is relative to the reference system itself. Without external force, it is stressed relative to the reference system itself, and we call this force inertial force. The reason why an object is subjected to inertial force in a non-inertial system is because it is not subjected to force in an inertial system. The inertial force of an object in a non-inertial system is a description of the force that an object is not subjected to in an inertial system. It is different descriptions of the same physical phenomenon that an object is subjected to inertial force in a non-inertial system and an object is not subjected to force in an inertial system.

Inertial system and non-inertial system describe the same physical phenomenon differently, but this description is equivalent.

They are equivalent because they are different descriptions of the same phenomenon. That is, the inertial force of an object in a non-inertial system is equal to that of an object in an inertial system, that is, the inertial force is equal to that of an object in an inertial system. F stands for inertial force in non-inertial system and F0 stands for unstressed force in inertial system, so F= F0. F=-ma, F0=0, inertial force F is non-zero, F0 is zero, and F= F0 shows that the definition of the starting point of force in inertial system and non-inertial system is different. Formula F= F0 is established because it describes the same phenomenon, and the inequality of quantities is due to the fact that inertial system and non-inertial system have their own definitions of F0, that is, there is no definition of force; It is caused by the different definitions of the starting point of force between inertial system and non-inertial system The reference system, namely inertial system and non-inertial system, defines the starting point of force according to itself. Because the motion state of inertial system and non-inertial system is different, the definition of the starting point of force will produce different quantities. When studying the motion of the earth's surface atmosphere, water, etc. Geostrophic deflection force is often used as inertial force. It is also very useful to study the motion of stars in space science, such as why asteroids are torn when they are close to Jupiter (the interaction of inertia force and gravity makes asteroids split) and why comets have declination when they are close to the sun.