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