This material should be familiar to the students, but sets the stage for the next lecture on relativistic conservation laws. It is important to emphasize that Newtonian momentum conservation requires the additional assumption that mass is conserved, but that these two conservation laws together then imply conservation of (kinetic) energy without additional assumptions.
It is worth going over both this lecture and the following one in advance in order to ensure a smooth presentation — the instructor should make sure to decide which way the reference frames are moving, and what notation to use for the quantities in each reference frame.