We went over a collision that was in two different parts assumed to be elastic and inelastic.
For an elastic collision, energy is conserved and momentum is conserved.
For an inelastic collision, only momentum is conserved.
So momentum is conserved in either situation.
You can derive equations by using those two ideas for an elastic collision (which takes a few steps). Or you can also use equations that are similar to the purpose of the ‘range equation’ seen previously in this class.
The work for the inelastic collision ends up being simpler.