Gravitational or inertial mass has an equivalent frequency according to the mass equation m·c^2 = h·f.
f = m·c^2/h=> kg·(m^2s^-2)/m^2·kg·s^-1=> s^-1 (Hz.)
The expression s^−1, the reciprocal of one second, is one hertz, it is a SI derived unit.
The unit of mass (m) is kg. Where there is mass there is energy and where there is energy there is mass.
The unit of energy (E) is Joule - which, in SI system, is kg⋅m^2⋅s^−2.
Therefore, mass m = E/c^2 (kg⋅m^2⋅s^−2/m^2⋅s^-2) = E/c^2 kg.
