A stone of mass m is tied to a string and is moved in a vertical circle of radius r making n rev/min. The total tension in the string when the stone is at the lowest point is ?

The free body diagram of the stone at the lowest point can be made
According to Newton’s second law of motion, the net force acting on the stone at this point is equal to the centripetal force, i.e.,
Fnet = T – mg = mv12 / R   ….(i)
Where, v1 = Velocity at the lowest point
The free body diagram of the stone at the highest point can be made also
Using Newton’s second law of motion, we have:
T + mg = mv22 / R    …(ii)
Where, v2 = Velocity at the highest point
It is clear from equations (i) and (ii) that the net force acting at the lowest and the highest points are respectively (T – mg) and (T + mg).