A stone of mass m tied to the end of a string revolves in a vertical circle of radius R. The net forces at the lowest and highest points of the circle directed vertically downwards are : [Choose the correct alternative

`T_1` and `v_1` denote the tension and speed at the lowest point. `T_2` and `v_2` denote corresponding values at the highest point.

Explanation

Alternative (a) is correct , note mg ` + T_2 = mv_2^2 // R , T_1 - mg = mv_1^2 // R`
The moral is : do not confuse the actual material forces on a body (tension, gravitational force, etc) with the effects they produce : centripetal acceleration `v_2^2//R or v_1^2//R` in this example.