A simple pendulum of length $l_1$ has periodic time $2.4$ s. Another simple pendulum of length $l_2 < l_1$, has periodic time $1.8$ s. The periodic time of simple pendulum of length $(l_1 - l_2)$ is nearly

Two springs of force constants '$2k$' and '$k$' are connected to a mass '$m$' as shown. Mass is displaced slightly to one side and released. The frequency of oscillation of the two springs-mass system is

Two oscillating simple pendulums with time periods $T$ and $\dfrac{4T}{3}$ are in phase at a given time. They will be again in phase after an elapse of time

A wire of length '$L$', diameter '$d$', density of material '$\rho$' is under tension '$T$' has fundamental frequency of vibration '$n_A$'. Another wire of length '$2L$', diameter '$3d$', density of material '$2\rho$' is vibrated under tension '$2T$', the fundamental frequency of vibration becomes '$n_B$'. The ratio $n_B : n_A$ is