What does inertia refer to in physics?

Inertia in physics specifically refers to the property of an object that determines its resistance to changes in its state of motion. This means that an object will remain at rest unless acted upon by an external force, and an object in motion will continue to move at a constant velocity in a straight line unless influenced by another force. This concept is rooted in Newton's first law of motion, which emphasizes that it takes a force to change the motion of an object.

The correct choice encapsulates this fundamental principle of inertia, highlighting the idea that an object's tendency is to maintain its current state, either at rest or in uniform motion. The other options focus on different aspects of motion or forces, such as the force applied to an object, the speed of movement, or the energy related to motion, but do not accurately define what inertia itself is. All of this reinforces why the first choice is the most accurate representation of inertia in the context of physics.