M.3.4 Hoop and Flask

This is a dramatic demonstration of inertia. A metal hoop of spring steel is balanced on the mouth of a flask, and a piece of chalk is balanced on the hoop, directly over the mouth of the flask. The object is to snatch the hoop away so the chalk falls into the flask. Tell your audience that this is a difficult demonstration and you need a couple of practice shots first. In your "practice shots" grab the hoop by its leading edge so the top is compressed upward ejecting the chalk wildly into the air. After telling the audience you are now ready to try it for real, grab the hoop by its trailing edge so its top is pulled out from under the chalk, dropping the chalk neatly into the flask! (From The Physics Teacher, Feb. 1982).