How does inertia relate to motion?

Inertia is fundamentally defined as the property of an object that causes it to resist changes in its state of motion. This means that an object at rest will remain at rest, and an object in motion will continue to move at a constant velocity unless acted upon by an external force. This characteristic of inertia is directly related to the mass of an object; the greater the mass, the greater the inertia. Therefore, inertia is not a force itself, nor does it define an object's speed or a calculation involving mass and velocity. Understanding inertia is critical in physics as it underlines the concept of Newton's First Law of Motion, which states that an object will remain in its current state of motion unless influenced by an outside force. This principle helps explain a variety of phenomena, from why we feel a jolt when a car suddenly brakes to how planets move in space.