Mass is generally defined in two different ways, both of which are traditional concepts in the realm of physics. The first definition, also called "invariant mass" or "rest mass," suggests that mass is an invariant quantity that is the same for every observer. This definition holds true when the body is at rest. Once the body begins to move, the measurement of mass changes.
The second definition, or what is called "relativistic mass," suggests that the velocity of the observer has an effect on the way mass is measured. In other words, relativistic mass is mass assigned to a body in motion. Relativistic mass is expressed as mr and invariant mass is expressed as m0. Relativistic mass increases with velocity and reaches infinity when the velocity approaches the speed of light, or c. Below are the equations for both masses:
mr = E/c2In the equations above, E is energy, p is momentum and c is the speed of light in a vacuum.
Relativistic mass is a term used when speaking about the total energy of a system. Relativistic mass is dependent on the observer's frame of reference. From the center of momentum frame, relativistic mass is the same as invariant mass. Invariant and relativistic masses are equal only when the center of mass of a system is at rest and there is no net momentum. In other words, there is zero velocity when relativistic mass equals invariant mass. The relativistic mass in any other frame contains the net kinetic energy of the body. The faster the body or system moves, the larger the relativistic mass.
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