When the bob is released from the horizontal position A, it will reach the equilibrium position O by moving along a circular path as shown in fig. above.

At the point O, the bob of the pendulum is acted upon by two, forces: its weight 'mg' and tension T in the string. The resultant of these forces provide the necessary centripetal force i.e.,

mv^{2}/l = T - mg or, T = mv^{2}/l + mg ...(i)

The velocity 'v' can be determined by applying the principle of conservation of energy, i.e.,

Kinetic energy of the bob at the point O

= Potential energy at the point A

⇒ 1/2 mv^{2} = mgl ⇒ v = √{2gl}

In the equation (i), substituting for v, we have

T = {m(√{2gl})^{2}}/{l} + mg = 3 mg