Heat or deformation cannot contribute to velocity here, as per the view of conservation on momentum. So how is it that momentum is conserved but kinetic energy is not given a perfectly inelastic collision? The two masses stick together. There is no intrinsic means of expressing lost due to deformation or heating assuming a perfectly inelastic collision considering the view conservation of momentum. Therefore, given the same collision (perfectly inelastic) considering conservation of energy, there can be no transfer of energy via vibrations (or there would have to be as much in the latter view). Thus, the only remaining mechanism to account for the decrease of kinetic energy of the body in motion would be an increase in volume of the system. No body with a rest mass can have a zero volume. So, when mass increases volume must also? (holding pressure constant) There cannot be deformation in this scenario or heat transfer, because there is no force to oppose the change in velocity of block at rest. As soon as the bullet applies force on the block it accelerates. Help?