Here's a breakdown:

* Momentum: A measure of an object's mass in motion. It's calculated by multiplying an object's mass (m) by its velocity (v): momentum (p) = m * v

* Closed system: A system where no external forces act upon it. In simpler terms, it means there's no momentum entering or leaving the system.

* Conservation: The total momentum of the system remains the same before, during, and after any interactions within the system.

Example:

Imagine two billiard balls colliding on a frictionless table (our closed system). Before the collision, each ball has its own momentum. When they collide, the balls exchange momentum, but the *total* momentum of the system (both balls combined) remains constant.

Key points to remember:

* Vector quantity: Momentum is a vector quantity, meaning it has both magnitude (amount) and direction.

* Internal forces: Interactions within the system (like collisions) don't affect the total momentum.

* External forces: If external forces act on the system (like friction), the total momentum will change.

The law of conservation of momentum has important applications in various fields, including:

* Rocket propulsion: Rockets work by expelling mass (fuel) in one direction, resulting in an equal and opposite momentum change for the rocket.

* Collisions: Understanding momentum conservation helps analyze collisions in car accidents, sports, and other scenarios.

* Particle physics: Conservation of momentum is a fundamental principle in the study of subatomic particles.