What is the length of the second's pendulum?

We know the time period (T) of the second pendulum is 2 s.

The time period $T= 2\pi \sqrt{\frac{l}{g}}$

Where l is the length of the pendulum, g is the acceleration due to gravity = 9.8 m/s².

$2 = 2\times 3.14\sqrt{\frac{l}{9.8 }}$

squaring on both sides we get 

$2^2 ={ 2}^2\times {3.14}^2{(\sqrt{\frac{l}{9.8 })}}^2$

$4 = 4 \times 9.8596\times \frac{l}{9.8}$
 

⟹ l = 0.994 m = 99.4 cm. 

Hence, the length of the second pendulum is 99.4 cm.