$A$ pendulum clock is placed inside an elevator. Under what condition does it show the correct time?

The time period of a simple pendulum will be doubled if we

$A$ light balloon filled with helium of density $\rho_{He}$ is tied to a light string of length $L.$ The string is tied to the ground forming an "inverted" simple pendulum (figure). If the balloon is displaced slightly from equilibrium as in figure and released, the period of the motion is. Take the density of air to be $\rho_{air}$. Assume the air applies a buoyant force on the balloon but does not otherwise affect its motion.

$A$ simple pendulum of length $L$ has mass $m$ and it oscillates freely with amplitude $A$. At the extreme position,its potential energy is (where $g$ is the acceleration due to gravity):

$A$ simple pendulum is oscillating without damping. When the displacement of the bob is less than maximum,its acceleration vector $\vec a$ is correctly shown in

The time period of a simple pendulum is $T$. When the length is increased by $10 \ cm$, its period is $T_1$. When the length is decreased by $10 \ cm$, its period is $T_2$. Then, the relation between $T, T_1$, and $T_2$ is