Section+5.4+-+Conservation+of+Momentum+in+2D

In order for one-dimensional collisions to have **conserved momentum**, the **net force on the system must be zero.** If the other forces that act on the system also add to zero or are so small that itself is negligible, then the net force of the system will be zero**.**

This reasoning still applies to collisions in two dimensions, thus they are analyzed using the same principles as collisions in one dimension: Conservation of momentum for all collisions for which the net force on the system is zero, and both conservation of momentum and conservation of kinetic energy if the collision is elastic. An example would be the collision of two air hockey pucks where friction is negligible ( Physics 12, Investigation 5.3.1.).

If there is a collision where there is no net force acting on the system **during the collision**, momentum is conserved, therefore: