সেকেন্ড দোলকের দৈর্ঘ্য চারগুণ করলে দোলনকাল গুন কত হবে?

Two particles executing SHM of the same amplitude and frequency on parallel lines side by side. They cross one another when moving in opposite directions each time their displacement is $\dfrac{\sqrt{3}}{2}$ times their amplitude. The phase difference between them is:

একজন বালিকা একটি দোলনায় বসে দোল খাচ্ছে। বালিকাটি উঠে দাঁড়ালে দোলনকালের কি পরিবর্তন হবে? 

The time period of a simple pendulum is air is $T$. Now the pendulum is submerged in a liquid of density $\dfrac{\rho}{16}$ where $\rho$ is density of the bob of the pendulum. The new time period of oscillation is.

The amplitude and the periodic time of a SHM are $5cm$ and $6s$ respectively. At a distance of $2.5cm$ away from the mean position, the phase will be