When a bike tire turns [b]one revolution[/b] the bike travels a distance equal to the circumference of the tire. If the tire has a [b]radius [/b]of 1 unit, then its [b]circumference [/b]is [math]C=2\pi r=2\pi\left(1\right)=2\pi[/math]. [br][br]As the tire rolls in this visualization, the circumference (shown in red) unrolls and sticks to the ground. The length of that unrolled piece of the circumference is the [b]arc length[/b] corresponding to a certain [b]fraction of a revolution[/b].