  We examine the validity of the hydrostatic equilibrium (HSE) assumption for
galaxy clusters using one of the highest-resolution cosmological hydrodynamical
simulations. We define and evaluate several effective mass terms corresponding
to the Euler equations of the gas dynamics, and quantify the degree of the
validity of HSE in terms of the mass estimate. We find that the mass estimated
under the HSE assumption (the HSE mass) deviates from the true mass by up to ~
30 %. This level of departure from HSE is consistent with the previous claims,
but our physical interpretation is rather different. We demonstrate that the
inertial term in the Euler equations makes a negligible contribution to the
total mass, and the overall gravity of the cluster is balanced by the thermal
gas pressure gradient and the gas acceleration term. Indeed the deviation from
the HSE mass is well explained by the acceleration term at almost all radii. We
also clarify the confusion of previous work due to the inappropriate
application of the Jeans equations in considering the validity of HSE from the
gas dynamics extracted from cosmological hydrodynamical simulations.
