The law you're looking for is Newton's Second Law of Motion in its more general form, which is often called the Conservation of Momentum.

Here's the breakdown:

* Newton's Second Law: This law states that the net force acting on an object is equal to the rate of change of its momentum. Mathematically:

* F = dp/dt

* Where:

* F is the net force

* p is the momentum

* t is time

* Conservation of Momentum: If the net force on an object is zero (no external forces acting on it), then the rate of change of momentum is also zero. This means the momentum remains constant.

Key Point: The Conservation of Momentum is a fundamental principle in physics and applies to both single objects and systems of multiple objects. It's a powerful tool for analyzing collisions, explosions, and other interactions where forces are involved.