Is Inertia a Force?

Inertia is not a force. Scientist Isaac Newton defined it as the property of a massive body that keeps it still if it is already still, or keeps it in motion if it is already in motion. Inertia is inextricably tied to mass, although it is a distinct quantity.

A force is the only thing capable of influencing the inertia of a body. If a force is applied and the mass moves, then that force has overcome the body's inertia. Alternatively, if the body was already moving and comes to a stop due to the force, the same thing has happened; the force has overcome the inertia.