 Wrap the wheel with string.
 Mark the beginning and end of this loop on the string.
 Extend it far enough and measure the length of the circumference corresponding to the wheel.
Measuring this dimension, you will find approximately 125.6cm, which is a little over 3 times its diameter. Let's see how to determine this length by a nonexperimental process.
You have probably heard of an old mathematical discovery:
Dividing the length of a circumference (C) by measuring its diameter (D), we always find a value approximately equal to 3.14.Like this:
The number 3,141592… corresponds in mathematics to the Greek letter (reads "pi"), which is the first reading of the Greek word perimeter. It is usually considered = 3,14.
Soon: Using this formula, we can determine the length of any circumference. We can now check with the formula the experimentally obtained wheel length. C = 2r C = 2. 3.14 · 20 C = 125.6 cm 
