Total momentum of a system is constant. When two hamster balls collide, the momentum is conserved. A collision is an event in which two or more objects approach and interact strongly for a brief period of time. If a 4 kilogram hamster with a velocity of 2 m/s collides with another hamster who is 3 kilograms and has a velocity of 5 m/s they would have 23 kg*m/s. After the collision however they still have the same amount of momentum because momentum is conserved. Initial momentum equals final momentum.