Britannica Sources used in this answer

Why is momentum conserved in an isolated system?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

A diagram of a cylindrical vessel containing gas, with a movable piston held down by a weight. Countless small dots beneath the piston represent gas particles in rapid, random motion, illustrating Bernoulli's model of gas pressure. — physics As conceived by Daniel Bernoulli in *Hydrodynamica* (1738), gases consist of numerous particles in rapid random motion. He assumed that the pressure of a gas is produced by the direct impact of the particles on the walls of the container.

In an isolated system, momentum is conserved because there are no external forces acting on the system as a whole to change its total momentum.12

This principle is rooted in Newton’s laws of motion. While internal forces—the forces that parts of a system exert on one another—may accelerate individual components, Newton’s third law dictates that these forces always occur in equal and opposite pairs. Consequently, when summing the forces acting within the system, these internal interactions cancel each other out, resulting in a net force of zero.1

Because the rate of change of momentum is equal to the net force applied, the absence of external forces means the total momentum of the system remains constant over time. This holds true regardless of the complexity of the system or the nature of the internal forces involved.1