The string of a pendulum of length l and bob of mass m is displaced through 90°….

Q:The string of a pendulum of length l and bob of mass m is displaced through 90°. Minimum strength of the string in order to withstand the tension should be.

(a) mg

(b) 2 mg

(c) 3 mg

(d) 4 mg

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Ans: (c)

At mean position body is in vertical equilibrium

$ T – \frac{m v^2}{l} = m g$ …(i)

$ v^2 = 2 g l $ …(ii)

from (i) & (ii)

$T – 2 m g = m g$

T = 3 m g