Question

what is dynamic inertia

Answer 1

Dynamic inertia is the inertia acting on a body whose direction keeps changing with the time.
Like if you're riding a bus, standing, and the bus turns at many different curves simultaneously, you will feel inertia acting on you from different directions. That's dynamic inertia.

Answer 2

dynamic inertia refers to the inertia of a body in uniform motion in the same straight line.