We investigate low-velocity, highly interpenetrating ("hard") head-on encounters between equal mass spherical galaxies by means of full N-body simulations. We compute the energy changes (both orbital and internal) caused by the collitions strength (the perturber mass-weighted inverse of the relative velocity at closes approach, Mₚ/Vₚ). Galaxies always gain energy in the encounters, but their rms velocity decreases after the collision, extra-energy being used to expand the galaxy rather than increasing its kinetic energy. The galaxies fall into bound orbits and energy rapidly if Vₚ < 2.75 σₚ, where σₚ is the rms velocity of the galaxy at maximum overlapping.