In a system, any interaction of particles constituting the system tends to change the particles' momentum. Still, the total momentum of the system remains constant in the absence of any external forces. The requirements for momentum conservation are – the system's total mass remains unchanged during the interaction, and the net external force acting on the system is zero. Such a system is called a closed system. The law of conservation of momentum states that the change in momentum due to particles' interaction in a closed system is zero. The momentum before interaction is equal to the momentum after interaction between the particles in a closed system. In the case of the rolling ball hitting the bowling pins, the initial momentum is only due to the ball as pins are at rest. When the ball hits the pins, they start moving, thus gaining momentum. However, the total momentum of the entire system is conserved as the ball slows down after hitting the pins.