Please do procedure I, then as many of the others as time allows. Collisions are a common experience in our lives-the collisions of billiard balls, the collisions of two football players, the collisions of cars. As early as the XVII century it was understood that collisions between hard bodies, like billiard balls, behaved differently from collisions like car wrecks, where the two masses stick together after the collision. A physical quantity is said to be conserved in a process if its value does not change even though other quantities are changing. An example of a conserved quantity is energy.
Linear momentum is conserved if the net force acting on an object is zero.
This follows from the equation which relates the change in momentum to the impulse given to the object. Clearly if the force is zero, the impulse is zero and the change in momentum is zero, hence the momentum remains constant. This same principle becomes more useful when it is applied to an isolated system of objects. A simple such system may consist of two particles, let us call them A and B, interacting with each other. Because the times during which the forces act are the same, it follows that the changes in momentum of the two particles are also equal and opposite, so that the total change in momentum is zero.