Definition

Noun: - (Physics) The mass of a body as measured when the body is at rest relative to an observer; an inherent property of the body. This is the mass of an object in its own frame of reference, where it is not moving. It is a fundamental, invariant property in physics, distinct from relativistic mass, which increases with velocity.

Usage

• Rest mass is a specific scientific term used primarily in the context of special relativity and particle physics. It describes the intrinsic mass of an object or particle.
• It is often contrasted with relativistic mass or total energy. The concept is crucial for understanding equations like (E = mc^2), where (m) represents the rest mass for an object at rest.

Examples

• The rest mass of an electron is approximately (9.109 \times 10^{-31}) kilograms.
• In special relativity, an object's rest mass remains constant regardless of its speed relative to an observer.
• Photons have zero rest mass, which is why they always travel at the speed of light.

Advanced Usage

• "Invariant mass": Another term often used synonymously with rest mass, especially in particle physics, to emphasize that this quantity is the same for all observers.
• The rest mass is the source of an object's gravitational attraction in Newtonian mechanics and is a key component in the stress-energy tensor in general relativity.

Variants and Related Words

• Mass (noun): The general property of matter that measures its resistance to acceleration. Rest mass is a specific type of mass.
• Relativistic mass (noun): The mass of an object as measured from a frame of reference in which it is moving. It depends on the object's velocity relative to the observer.
• Invariant mass (noun): A synonym for rest mass, highlighting its property of being unchanged by the observer's frame of reference.

Synonyms

• Intrinsic mass: Emphasizes that the mass is an inherent property of the body.
• Proper mass: A less common synonym used in some physics contexts.

Related Phrases and Concepts

• Mass-energy equivalence: The concept, expressed by (E = mc^2), that the rest mass of a body is a measure of its energy content.
• Rest energy: The energy equivalent of an object's rest mass, calculated as (E0c^2).
• "At rest": A prerequisite state for measuring rest mass; the object's velocity is zero relative to the observer.

Noun

1. (physics) the mass of a body as measured when the body is at rest relative to an observer, an inherent property of the body