Students are first able to ?unroll? the circumference of a circle to form a straight line and get a feel for the length of a circumference (an alternative to the traditional method of measuring strings around cans.). Secondly they can slide several identical circles to estimate how many diameters ("three and a bit") make up the circumference.
This is followed by measuring c and d on the screen and dividing their lengths. They can record in a spreadsheet the diameter and circumference lengths for various circles. Dividing values of c by values of d leads to an understanding that circumference and diameter are proportional, and the discovery of the value of π.
The next step is to compare radius with circumference and see that c=2πr.
Several ?real-world? problems involving circumferences are included and also an extension problem involving rolling one circle around the inside or outside of a larger circle.
This activity forms part of the OxBox, published by Oxford University Press to support their course 'Oxford GCSE Maths'. http://www.oup.com/oxed/secondary/2010gcse/