Jerome should always keep momentum constant for each object after the objects collide and bounce apart.
According to the law of conservation of momentum, the total momentum of a closed system remains constant if no external forces act upon it. This means that while individual objects may change their velocity or direction after the collision, the overall momentum before the collision must equal the overall momentum after the collision.
Mass is also constant for a given object, but it is not a variable that changes as a result of the collision; rather, momentum (which is the product of mass and velocity) is the key quantity that is conserved during the collision process.