Time period of a simple pendulum is T inside a lift when the lift...

When lift is stationary

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

When lift is moving upwards

$⇒$ Pseudo force acts downwards

$⇒ {g}_{eff} = g + \dfrac{g}{2} = \dfrac{3g}{2}$

$⇒$ New time period

${T}^{'} = 2\pi\sqrt{\dfrac{L}{{g}_{eff}}} = 2\pi\sqrt{\dfrac{2L}{3g}}$

${T}^{'} = \sqrt{\dfrac{2}{3}}T$