Inertia is the resistance of any physical object to any change in its state of motion, including changes to its speed and direction. It is kind of counterforce which is resisting the change of speed. But when there is this counterforce which is at the same level as force causing the acceleration, why is acceleration actually possible? Is this because there is some time lag between force causing acceleration and inertia counterforce? Have there been some experiments about this topic? Or is there some other explanation? But what I know, cause of inertia itself have not been explained yet.