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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.

Answers (1)

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

                                        Laws of motion,   20275

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\ =\ \frac{mv1^2}{r}

Where, v = Velocity at the lowest point

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

                                                      Laws of motion,   20275 (2)

Using Newton's law of motion we have :

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

Where v = Velocity at the highest point

It is clear from the above equations that the net force acting at the lowest and the highest points are respectively (T – mg) and (T + mg).

Posted by

Devendra Khairwa

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