Penny and Coat Hanger
Purpose
A penny balanced on the hook of an elongated coat hanger stays in place when the hanger is swung in a vertical circle around your finger. Students see that the required centripetal acceleration keeps the penny pressed against the hanger even at the top of the arc, where gravity alone would cause it to fall.
Figure 1:
Elongated wire coat hanger bent into a diamond shape. The hook end at the bottom has been filed flat and roughened to help balance the penny. The hanger is held at the top and swung around the finger.