The purpose of this applet is to allow the user to change the size of a circle's diameter and look for a relationship between the length of any circle's diameter and its circumference. Drag Point B to different places and record your findings.
I've updated it by simply enlarging the size of points A and B to make it easier for my students on iPod Touches to drag those points. All credit for this applet goes to jmamer.

If you know a circle's diameter how can you predict its circumference?
If you know a circle's radius how can you predict its circumference?
If you know a circle's circumference how can you predict it diameter or radius?