Define the unit of mass in nuclear physics. Explain the measurement of the mass of a body.

(A) In nuclear physics,the mass of an atom and nucleus is very small; hence,the unified atomic mass unit $(u)$ is established for expressing mass.
$1$ unified atomic mass unit $(1 u) = \frac{1}{12}$ of the mass of a carbon-$12$ isotope $({ }_{6}^{12} C)$ atom,including the mass of electrons,which is equal to $1.66 \times 10^{-27} \ kg$.
Generally,the mass of a body is measured using a common physical balance.
For large masses in the universe (like planets or stars),the measurement is based on Newton's universal law of gravitation:
$m = \frac{F r^2}{G M_e}$
To measure very small masses (like atoms),a mass spectrometer is used. In this instrument,the radius of the trajectory is proportional to the mass of the charged particle moving in a uniform electric and magnetic field.