
17. Mass/Weight Paul distinguishes between mass and weight in a video from his classroom, then breaks strings attached to a ball in ways that clarify the distinction. 

18. Acceleration
Units Paul clarifies the equivalence of m/s2 and N/kg, and leads to Earth's gravitational field g. 

19. Newton's Second
Law A video of Paul in the classroom is followed up with a discussion of force, mass, and the acceleration of freelyfalling rocks. 

20. Skydiver Problem Paul poses and solves a problem involving Suzie Skydiver, who dives and attains terminal velocity. 