The length of the arc is proportional to the degree measure it in encompasses. Since there are 360 degrees in a circle and the arc is 120 degrees, the arc's length will be of the circle's length (circumference).
The circumference of a circle is given by , where is the radius of the circle. Therefore, the circumference of the circle is . As we found earlier, the length of the arc is of this circumference. Therefore, the arc's length is .