  By analyzing the many-body problem for non-relativistic electrons strongly
coupled to photon modes of a microcavity I derive the exact momentum/force
balance equation for cavity quantum electrodynamics. Implications of this
equation for the electron self-energy and the exchange-correlation potential of
quantum electrodynamic time-dependent density functional (QED-TDDFT) are
discussed. In particular I generalize the concept of $\Phi$-derivability to
construct approximations which ensure the correct momentum balance. It is shown
that a recently proposed optimized effective potential approximation for
QED-TDDFT is conserving and its possible improvements are discussed.
