  A non-linear gravitational wave imparts gravitational acceleration to all
particles that are hit by the wave. We evaluate this acceleration for particles
in the pp-wave space-times, and integrate it numerically along the geodesic
trajectories of the particles during the passage of a burst of gravitational
wave. The time dependence of the wave is given by a Gaussian, so that the
particles are free before and after the passage of the wave. The gravitational
acceleration is understood from the point of view of a flat space-time, which
is the initial and final gravitational field configuration. The integral of the
acceleration along the geodesics is the analogue of the Newtonian concept of
work per unit mass. Surprisingly, it yields almost exactly the variation of the
non-relativistic kinetic energy per unit mass of the free particle. Therefore,
the work-energy relation $\Delta K = \Delta W$ of classical Newtonian physics
also holds for a particle on geodesics in the pp-wave space-times, in a very
good approximation, and explains why the final kinetic energy of the particle
may be smaller or larger than the initial kinetic energy.
