  We study the reaction of a globular star cluster to a time-varying tidal
perturbation (gravitational shock) using self-consistent N-body simulations and
address two questions. First, to what extent is the cluster interior protected
by adiabatic invariants. Second, how much further energy change does the
postshock evolution of the cluster potential produce and how much does it
affect the dispersion of stellar energies. We introduce the adiabatic
correction} as ratio of the energy change, <dE>, to its value in the impulse
approximation. When the potential is kept fixed, the numerical results for the
adiabatic correction for stars with orbital frequency \omega can be
approximated as (1 + \omega^2 \tau^2)^{-\gamma}. For shocks with the
characteristic duration of the order the half-mass dynamical time of the
cluster, \tau < t_{dyn,h}, the exponent \gamma = 5/2. For more prolonged
shocks, \tau > 4 t_{dyn,h}, the adiabatic correction is shallower, \gamma =
3/2. When we allow for self-gravity and potential oscillations which follow the
shock, the energy of stars in the core changes significantly, while the total
energy of the system is conserved. Paradoxically, the postshock potential
fluctuations reduce the total amount of energy dispersion, <dE^2>. The effect
is small but real and is due to the postshock energy change being statistically
anti-correlated with the shock induced heating. These results are to be applied
to Fokker-Planck models of the evolution of globular clusters.
