
Given the length of the pendulum is ℓ=10m.
The initial potential energy of the pendulum at the horizontal position is given by
U=mgℓ...(1)
The final kinetic energy of the pendulum at the lowermost position is given by
K=21mv2...(2)
So, from equations (1) and (2), it follows that
109mgℓ=21mv2⇒109×10×10=21v2⇒v2=180⇒v=180=65ms−1