Consider a body of mass 'm' tied to a string and rotated in a vertical circle of radius 'r'. The velocity (speed) of the body keeps changing. It is maximum at the bottom and a minimum at the top.
Let u be the speed of the particle at the bottom
Let v be the speed at some point
By the principle of conservation of energy
Where (h is the vertical height of P above the lowest point)
Let T be the tension in the string.
Tension at the highest point is obtained by putting h = 2r in
Tension at the bottom of the circle is obtained by putting h = 0.
Tension at the highest point is given by putting h=2r
TL - TH = 6mg