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Mass (m) ○◂｜Definition｜1st｜20251119205401-00-⌔
Mass
In physics, mass is an intrinsic positive physical quantity of a body, which measures its resistance to acceleration. In modern physics, it is generally defined as the strength of an object’s gravitational attraction to other bodies — as measured by an observer moving along at the same speed. The unit of mass in the International System of Units (SI) is the kilogram (kg).
Mass was traditionally thought to be related to the amount of matter contained in a physical body, until the emergence of electromagnetic theory1 at the end of the 19th century (see electromagnetic mass), and later the advent of special relativity in 1905. It has been observed experimentally that different atoms and different elementary particles, theoretically containing the same amount of matter, nonetheless have different masses, in accordance with the predictions of special relativity. Since 1905, mass can also be understood as a measure of the energy content of a body (see E=mc²).
Mass can be experimentally defined as a measure of the body’s inertia, meaning the resistance to acceleration (change of velocity) when a net force is applied.2
In physics, mass is not the same as weight, even though mass is often determined by measuring the object’s weight using a spring scale, rather than balance scale comparing it directly with known masses. An object on the Moon would weigh less than it does on Earth because of the lower gravity, but it would still have the same mass. This is because weight is a force, while mass is the property that (along with gravity) determines the strength of this force.
In the Standard Model of physics, the mass of elementary particles is believed to be a result of their coupling with the Higgs boson in what is known as the Brout–Englert–Higgs mechanism.3
Printed 2026-06-28.
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Link to original Footnotes
The Feynman Lectures on Physics, Volume II, Chap. 28: Electromagnetic Mass ↩
Bray, Nancy (28 April 2015). “Science”. NASA. Archived from the original on 30 May 2023. Retrieved 20 March 2023. Mass can be understood as a measurement of inertia, the resistance of an object to be set in motion or stopped from motion. ↩
“The Higgs boson”. CERN. 3 April 2024. Retrieved 9 April 2024. ↩
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