Q5.26 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]                Lowest Point                                Highest Point    (a)    $mg - T_1$                                        $mg + T_2$​​​​​​    (b)    $mg + T_1$                                        $mg - T_2$    (c)    $mg + T_1 - (mv_1^2))/R$           $mg - T_2 + (mv_1^2))/R$    (d)    $mg - T_1 - (mv_1^2))/R$          $mg + T_2 + (mv_1^2))/R$$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.

The FBD of stone at the lowest point is shown below :

Using Newton's law of motion,

$T\ -\ mg\ =\ \frac{mv^2}{r}$

The FBD of stone at the highest point is given below :

Using Newton's law of motion we have :

$T\ +\ mg\ =\ \frac{mv^2}{r}$

Thus option (a) is correct.

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