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.