Pronunciation: [ɹˈɛst mˈas] (IPA) 
Rest mass is a term used in physics to refer to the mass of an object when it is at rest. The spelling of this word is phonetically represented as /rɛst mæs/, which means that the first sound of the first syllable is the short e sound, the second syllable is pronounced with the æ vowel sound as in the word "cat". The word "mass" is pronounced with the æ sound as well, and both syllables are pronounced with stress at the same level.
Rest mass refers to the intrinsic property of matter that measures its resistance to acceleration. It is defined as the mass of an object or particle when it is at rest, meaning that it is not moving with respect to an observer or frame of reference. The concept of rest mass is fundamental in understanding the behavior and nature of particles.
Rest mass is a fundamental property of particles in the subatomic world, such as electrons, protons, and neutrons. It is different from relativistic mass, which takes into account the additional mass acquired by an object as its velocity approaches the speed of light according to Einstein's theory of relativity.
The rest mass of a particle is considered invariant, meaning it does not change regardless of the observer's frame of reference. This property holds true even when the particle is in motion, as relativistic effects alter only the relativistic mass. The rest mass is often used in scientific equations to calculate energy, momentum, and other physical quantities.
The unit of rest mass is typically measured in kilograms or its derivatives in the metric system. In particle physics, the concept of rest mass plays a crucial role in the study of elementary particles and their interactions in the subatomic world. Understanding the rest mass of particles provides insights into their behavior, properties, and contributions to the fundamental forces and structures of the universe.
The term "rest mass" was coined in the field of physics to refer to the mass of an object when it is at rest. The word "rest" originates from the Old English word "ræst", which means "a period of inactivity or repose". It is derived from the Proto-Germanic word "rastō", meaning "rest" or "remaining". The word "mass" comes from the Latin word "masse", which originally referred to a lump or a physical quantity of matter. The etymology of "rest mass" reflects the understanding that an object's mass is distinct from its energy and is independent of its motion.
