The swinging sphere experiences two forces : The gravitational force and the tension of the thread. Now, it is clear from the condition, given in the problem, that the moment of these forces about the vertical axis, passing through the point of suspension Nz = 0. Consequently, the angular momentum Mz of the sphere relative to the given axis (z) is constant.
Thus
mv0(lsinθ)=mvl (1)
where m is the mass of the sphere and v is its velocity in the position, when the thread forms an angle π/2 with the vertical. Mechanical energy is also conserved, as the sphere is under the influence if only one other force, i.e. tension, which does not perform any work, as it is always perpendicular to the velocity.