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.,
mv2/l = T - mg or, T = mv2/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 mv2 = mgl ⇒ v = √{2gl}
In the equation (i), substituting for v, we have
T = {m(√{2gl})2}/{l} + mg = 3 mg