Why Momentum is Always Conserved: Newton's Third Law

⚙️ Why Momentum is Always Conserved: Newton's Third Law

Conservation of momentum is not a separate law — it follows directly from Newton's Third Law. When two objects collide, they exert equal and opposite forces on each other (Third Law). These forces act for the same time interval. Equal forces × equal time = equal and opposite changes in momentum. Therefore, any increase in momentum of one object is exactly matched by a decrease in the other. Total momentum is unchanged.

This is why momentum conservation applies to ALL collisions and explosions, regardless of the forces involved — provided there are no external forces (a "closed system"). In practice, friction with the ground is an external force, so real systems are only approximately closed.

The vector nature of momentum is crucial for explosions: when an object at rest explodes into two pieces, one goes left and one goes right with equal and opposite momenta, summing to zero (matching the initial momentum of zero).