This question tests the relationship between arc length, radius, and angle in radian measure. The formula θ = l/r is derived from the definition that in a unit circle, an arc of length 1 unit subtends an angle of 1 radian at the centre. For any circle of radius r, an arc of length l subtends an angle θ = l/r radians at the centre. The pendulum length acts as the radius, and the arc described by the tip is the arc length.