What does inertial mass measure?

Inertial mass is defined as a measure of an object's resistance to changes in its state of motion when a force is applied. This concept is rooted in Newton's second law of motion, which states that the force acting on an object is equal to its mass times its acceleration (F = ma). When an external force is applied to an object, it will accelerate depending on its mass; heavier objects, or those with greater inertial mass, require more force to achieve the same acceleration as lighter objects.

The other options do not accurately define inertial mass. While the amount of matter in an object may be related to its mass, it specifically refers to the concept of "gravitational mass," not the inertial mass associated with motion and force. Weight pertains to the force of gravity acting on an object and is distinct from inertial mass as it involves gravitational acceleration. Finally, speed is a measure of how fast an object moves, which does not directly relate to an object's resistance to changes in motion.