Question

What is the cause of inertia?

Answer 1

Resistance, read below...

Explanation:

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 `0N`, the object will remain stationary. If the resultant force on a moving object is `0N` it will carry on moving at the same velocity" (same speed and direction, as velocity, is a vector, which has speed and direction).

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 `0N`, a stationary object will remain stationary, and a moving object will remain moving with the same velocity, but resistance actually causes it.

Although there is an interesting forum here which goes against my point... https://www.physicsforums.com/threads/what-is-the-cause-of-inertia.410666/