Mass is a measure of the amount of matter in an object. It is not the same as weight. Weight is the force of gravity on an object, and it changes depending on where you are. Mass does not change. An astronaut weighs less on the moon, but their mass is the same. Mass is measured in kilograms. The international prototype kilogram, a platinum-iridium cylinder, defined the kilogram for over a century. In 2019, the definition changed to one based on Planck's constant, making it more precise and universal.

Mass has two important properties. Inertial mass is the resistance of an object to changes in motion. The more mass an object has, the more force is needed to accelerate it. Gravitational mass is the strength of an object's gravitational attraction to other objects. Experiments have shown that inertial mass and gravitational mass are equal to within one part in a trillion. That equivalence is the foundation of Einstein's general theory of relativity. Mass can be converted into energy, and energy into mass, according to E = mc². That is the principle behind nuclear reactions and particle accelerators.

Mass facts

• Definition. Amount of matter in an object.
• Unit. Kilogram (kg).
• Inertial mass. Resistance to acceleration.
• Gravitational mass. Strength of gravitational attraction.
• Equivalence. Inertial and gravitational mass are equal.

Mass is not the same as weight. Weight is a force. Mass is a property of matter.