The document presents 6 key formulae relating to forced vibration. Formula 1 gives the magnification factor or amplitude relation for forced vibration as a function of excitation frequency and damping ratio. Formula 2 gives the phase lag between displacement and force vectors. Formula 3 gives the non-dimensional amplitude relation for vibration due to unbalance as a function of excitation frequency and damping ratio. Formula 4 gives the non-dimensional displacement transmission relation for forced vibrations due to base excitation. Formula 5 gives the relation for force or displacement transmissibility. Formula 6 gives the relation for phase lag between motion of the mass and motion of the support or between applied and transmitted force.