Question

The principle of conservation of linear momentum can be strictly applied during a collision between two particles provided the time of impact is

Answer 1

The short answer is that momentum is always conserved.

What you may be noticing is something weird, namely that in perfectly elastic collisions, momentum and energy are conserved, whereas in an inelastic collision, momentum is conserved, but energy is not.

That's not entirely accurate either, though, because of course, energy is always conserved, too.