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.

End Question:

If you want to tighten a loose hammerhead onto its handle, it’s a good idea to slam the bottom of the handle against a firm surface as shown, rather than the other way around. Is it gravity or inertia that accounts for the sudden stop tightening the hammerhead? H. C. (How come?)