3.3.4—That, while momentum of a system is always conserved in interactions between

If the system is isolated, momentum remains conserved in an inelastic interaction, but total kinetic energy after is smaller because energy is transferred to deformation, heat or sound.

Check whether objects separate or stick, then use the momentum equation. Track the lost kinetic energy as a change of energy store, not as lost total energy.

A lump of clay hitting a stationary block and sticking has one common final velocity; the difference in kinetic energy becomes internal energy.

“Kinetic energy is not conserved” does not mean energy disappeared, and inelastic does not mean momentum is unbalanced.