Description: Mass is the amount of matter a body contains. The number, type, and density of atoms that make up the body combine to determine the mass of that body. In everyday speech mass and weight are often used interchangeably but in physics they are not the same. Mass remains the same regardless of location while weight depends on the local gravitational field.

There are two concepts of mass: "gravitational mass" and "inertial mass". "Inertial mass" is the property of matter that causes it to resist acceleration; it is the mass in Newton's second law. We see that acceleration is inversely proportional to mass. Hence the more massive a body the less it accelerates for a given force.

"Gravitational mass" is the property of matter that causes it to exert and experience the gravitational force. The more massive a body, the greater force due to gravity it feels.

Albert Einstein asserted that these two mass concepts are identical via his "principle of equivalence". This is a fundamental concept in physics.

The kilogram (kg) is the internationally recognized unit of mass measurement (other units can be used such as: grams, milligrams, ton, ounce, and pound), and mass is represented by the symbol m. Mass is a scalar quantity and is a form of energy. As such, it can be transformed into other forms of energy, such as during nuclear reactions, and it is not strictly conserved. Massless particles, such as photons, the particles that carry electromagnetic radiation, exist in nature.

Related Terms:

• Dark Matter
• Electromagnetic Radiation
• Newton's Laws of Motion
• Nuclear Fission
• Nuclear Fusion
• Photon

Term and definition status: This term and its definition have been approved by a research astronomer and a teacher

The OAE Multilingual Glossary is a project of the IAU Office of Astronomy for Education (OAE) in collaboration with the IAU Office of Astronomy Outreach (OAO). The terms and definitions were chosen, written and reviewed by a collective effort from the OAE, the OAE Centers and Nodes, the OAE National Astronomy Education Coordinators (NAECs) and other volunteers. You can find a full list of credits here. All glossary terms and their definitions are released under a Creative Commons CC BY-4.0 license and should be credited to "IAU OAE".