If now the arc HI is rotated completely about the line PS to form a ring of width HI and radius IQ, the length of the ring is 2 π · IQ and its area is 2 π · IQ · IH. The component of the force due to this ring on the particle at P in the direction PS becomes ∝ ∝ I H ⋅ ⋅ I Q ⋅ ⋅ P Q P I 3 .