This GeoGebra app shows the relationship s = rθ.
One activity I get students can do is to look at the value of θ when the arc length s is equal to the radius r. This would give the definition of the radian, which is the angle subtended at the centre of a circle by an arc equal in length to its radius.
Mathematics defines the constant π as the ratio of a circle’s circumference to its diameter. This can also be shown in the app, although you need to drag the moving point to a point just short of one full revolution.
![](https://physicslens.com/wp-content/uploads/2020/05/definition-of-pi.png)