Question

The length of a simple pendulum which makes 20 oscillations in 36 seconds (at place where the acceleration due to gravity $9.75 m/s^2$ ) is

• A. 70 cm
• B. 80 cm
• C. 90 cm
• D. 60 cm

Answer 1

Answer: (B)

80 cm

Answer 2

Given $g = 9.75 m/s^2$
$T= \frac{36}{20} = 1.8 sec$
Using, $L = \frac{gT^2}{4 \pi^2} = \frac{9.75 \times (1.8)^2}{4 \times 10} \:\:\: \:\: ( \because \, \pi^2 \approx 10)$
= 0.79 m = 79 cm $\approx$ 80 cm