Thispaperpresentsatheoreticalmodelonthenormal(head-on)collisionbetweensoft-spheresonthebasisofelasticloadingoftheHertzcontactforcompressionprocessandanonlinearplasticunloadingforrestitutionone,inwhichtheparametersallaredeterminedintermsofthematerialandgeometriconesofthespheres,andthebehaviorsofperfectelastic,inelastic,andperfectplasticcollisionsappearedintheclassicalmechanicsarefullydescribedonceavalueofcoefficientofrestitutionisspeci...