Defining Two Types of Mass
Inertial mass is an object's resistance to being accelerated by a force, as described by Newton's second law (F=ma). Gravitational mass determines the strength of the gravitational force an object exerts and experiences, as described by Newton's law of universal gravitation.
Section 2: Resistance vs. Attraction
The core difference lies in their conceptual roles. Inertial mass is a measure of an object's inertia—how much it 'fights' a change in its state of motion. In contrast, gravitational mass is a measure of how an object participates in gravitational interactions, either by attracting other objects or by being attracted by them.
Section 3: A Practical Example
Imagine an astronaut in deep space, far from any significant gravity. If they try to shake a massive asteroid back and forth, they will find it very difficult. This difficulty is due to the asteroid's large inertial mass. If they were to measure the gravitational pull between their ship and the asteroid, they would be measuring its gravitational mass.
Section 4: The Equivalence Principle
Crucially, experiments have shown that inertial mass and gravitational mass are numerically equivalent for any object. This remarkable fact, known as the Equivalence Principle, is not explained by Newtonian physics but is a foundational pillar of Albert Einstein's theory of general relativity.