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Mass
The mass m is the degree to which a particle maintains in its movement.
Explanation
In SI units, the mass is defined in kilogram (symbol kg), by setting the Planck constant h to
Details
If a particle would be at rest, you have the rest mass m0. Near the speed of light, particles have a higher mass. This is called the relativistic mass mv.
Mass and energy bend spacetime, and affect the movement of particles, including light. Usually you write the mass as m if it is clear what mass is involved anyway.