What does inertia refer to in physics?

In physics, inertia refers to the property of an object that resists changes in its state of motion. This means that an object will remain at rest or move in a straight line at a constant speed unless acted upon by an external force. The concept of inertia is closely related to mass; the greater the mass of an object, the greater its inertia, which means it will resist acceleration more than an object with less mass. This principle is fundamental to Newton's first law of motion, which states that an object will not change its velocity unless acted on by a net external force. Understanding inertia is crucial in various fields of physics and engineering, as it helps predict how objects will behave under different conditions.

In the context of the other choices, while inertia is related to mass, it is specifically about resistance to change in motion rather than simply measuring mass or describing speed and direction.