Question

What is a seconds' pendulum?

Answer 1

Definition of seconds pendulum: a pendulum requiring exactly one second for each swing in either direction or two seconds for a complete vibration and having a length between centres of suspension and oscillation of 99.353 centimetres at sea level in latitude 45 degrees

Since the pendulum reaches its extreme position twice in one oscillation, the time period of a seconds pendulum is 2 seconds.

The time period of a simple pendulum of length L is

$T=2\pi \sqrt{\frac{L}{g}}$

Put T= 2 seconds
g = 9.81 $m{s}^{-2}$

$L=\frac{T^{2}g}{4{\pi }^2}$

$L=\frac{2^2\times 9.8}{4{\pi }^2}=0.994m$

On earth, the second's pendulum has a constant length of approximately 1 m.