Physical Science: Motion and Stability: Forces and Interactions - Evidence Statement 2
HS-PS2-2: Use mathematical representations to support the claim that the total momentum of a system of objects is conserved when there is no net force on the system.
[Clarification Statement: Emphasis is on the quantitative conservation of momentum in interactions and the qualitative meaning of this principle.] [Assessment Boundary: Assessment is limited to systems of two macroscopic bodies moving in one dimension.]
Source: http://www.nextgenscience.org/
For more specific information, including ways this performance expectation can achieve three-dimensional learning, please see this NGSS Evidence Statement.