Roll a wheel once
Press Roll one turn. The wheel's orange rim unrolls onto the ground as it goes, and the dot shows when one full turn is done. Then change the size of the wheel and roll again. Each roll adds a row to the table. Look down the last column.
| diameter | rolled in one turn | rolled ÷ diameter |
|---|
What's going on
In one turn, every bit of the rim touches the ground once. So the distance rolled is exactly the distance round the wheel, which is called the circumference.
The blue blocks under the ruler are each one diameter long (the diameter is the distance straight across the wheel). Three of them never quite reach the end of the orange strip. There is always a little bit left over, and it is the same share of a diameter every time.
A bigger wheel rolls farther, but the distance divided by the diameter does not change. It is 3.14159… for a coin, a bike wheel and the Earth. That number is called π (say "pie"), so circumference = π × diameter.
Sheet: Roll a wheel once