Which physical quantity conserved during collision

Linear momentum of an isolated system is conserved in all types of collisions (elastic, inelastic, perfectly inelastic). That is, Σp_before = Σp_after when net external force is zero. Kinetic energy is conserved only in elastic collisions; it decreases in inelastic ones (some energy converts to heat, sound, deformation).

In answers, write the momentum conservation statement, mention the isolation condition, and contrast KE behavior across collision types. This highlights both the universal conservation (momentum) and the conditional one (kinetic energy).