Definition of Rest Mass
Rest mass, also known as invariant mass, is the intrinsic mass of an object or particle as measured by an observer when the object is at rest relative to that observer. Unlike relativistic mass, which increases with velocity, rest mass is a fundamental and constant property of an object, independent of its motion.
Key Principles of Rest Mass
In classical physics, mass is often considered solely as rest mass. However, in Einstein's theory of special relativity, the concept becomes crucial because an object's total energy (E) is related to its rest mass (m₀) and momentum (p) by the equation E² = (m₀c²)² + (pc)², where c is the speed of light. This equation highlights that rest mass is a form of intrinsic energy.
Practical Example of Rest Mass
For everyday objects like a baseball, its rest mass is simply the mass you measure on a scale. When that baseball is thrown, its relativistic mass slightly increases due to its kinetic energy, but its *rest mass* remains unchanged. For fundamental particles like an electron, its rest mass is a fixed value that contributes to its energy even when it's stationary.
Importance in Modern Physics
Rest mass is a cornerstone of modern particle physics. It explains why some particles, like photons, are massless at rest (m₀=0) and thus must always travel at the speed of light, while particles with non-zero rest mass, like electrons and protons, can be accelerated but never reach the speed of light.