Mechanical energy E of particle, system or object is explained as the sum of kinetic energy K and potential energy U, i.e. E = K + U. It is a scalar quantity having dimensions [ML2T-2] and SI unit joule.
(i) It relays on frame of reference.
(ii) A body may have mechanical energy without having either kinetic energy or potential energy. Though, if both potential and kinetic energies are zero, mechanical energy will be zero. The result may or may not be true, i.e., if E = 0 either both KE and PE are zero or PE negative and KE positive such that KE + PE = 0