In physics, the principle of conservation of momentum states that the total momentum of a closed system remains constant before and after a collision or explosion. Momentum is a vector quantity, which means it has both magnitude and direction. When objects collide, the total momentum of the system is conserved, meaning that the total initial momentum is equal to the total final momentum. This principle can be derived from Newton's third law of motion, which states that for every action, there is an equal and opposite reaction.

To understand the conservation of momentum, let's consider an example. Imagine a game of billiards where a cue ball strikes a stationary ball. Before the collision, the cue ball has an initial momentum in a certain direction, and the stationary ball has zero momentum. After the collision, the cue ball will transfer some of its momentum to the stationary ball, causing it to move. However, the total momentum of the system before and after the collision remains the same.

This principle has important applications in various fields. One notable application is rocket propulsion systems. When a rocket propels itself forward, it expels hot gases at high speeds in the opposite direction. According to the conservation of momentum, the momentum of the expelled gases is equal and opposite to the rocket's momentum. This creates a forward thrust that propels the rocket. Another example is in the sport of figure skating. Skaters use their arms to extend or retract, changing the distribution of their mass and angular momentum while performing spins. By applying conservation of momentum, skaters can control their rotational speed and balance effectively.