Unit of measurement
Natural systems

While the above systems of units are based on arbitrary unit values, formalised as standards, natural units in physics are based on physical principle or are selected to make physical equations easier to work with. For example, atomic units (au) were designed to simplify the wave equation in atomic physics.

Some unusual and non-standard units may be encountered in sciences. These may include the solar mass (2 × 10 kg), the megaton (the energy released by detonating one million tons of trinitrotoluene, TNT) and the electronvolt.
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