On This Page
Overview
Conservation of momentum is one of the major conservation laws in mechanics. It is especially useful for collisions, explosions, recoil, and interactions where internal forces are complicated.
Isolated System
An isolated system has no external net force acting on it, or external forces are negligible during the time interval studied.
Internal and External Forces
Internal forces act between objects within the system. External forces come from outside the system and can change total system momentum.
Before and After Analysis
Momentum conservation problems often compare total momentum before an interaction with total momentum after it.
Collisions
Momentum is conserved in both elastic and inelastic collisions when the system is isolated. Kinetic energy is conserved only in elastic collisions.
Explosions and Recoil
If a system begins at rest and separates into parts, the momenta of the parts add to zero when no external net force acts.
Vector Nature of Momentum
Momentum is a vector, so conservation must account for direction in one, two, or three dimensions.
Common Mistakes
A common mistake is applying conservation to a system with a significant external net force. Another is confusing momentum conservation with kinetic energy conservation.
Applications
Conservation of momentum is used in crash analysis, sports, rocket motion, particle physics, explosions, recoil, and impact studies.
Why This Matters in Physics
Conservation of momentum shows how motion is redistributed during interactions.