What is the cause of inertia?
Resistance, read below...
Since Inertia is "The tendency for motion to remain unchanged", this simply means the measurement that an object will keep moving at a constant velocity, direction, speed, or at rest.
As Newton's first law states "If the resultant force on a stationary object is
Therefore the cause of Inertia is Resistance, as Inertia is the resistance for any moving or stationary object to change its state of motion, or as again previously stated as "The tendency for motion to remain unchanged". Force changes Inertia, as previously stated in Newton's first law if a resultant force is
Although there is an interesting forum here which goes against my point... https://www.physicsforums.com/threads/what-is-the-cause-of-inertia.410666/