A spring mass system (mass $m,$ spring constant $k$ and natural length $l$ ) rests in equilibrium on a horizontal disc. The free end of the spring is fixed at the centre of the disc. If the disc together with spring mass system rotates about it's axis with an angular velocity $\omega ,\left(k>>m{\omega }^2\right)$ the relative change in the length of the spring is best given by the option:

The displacement time graph of a particle executing SHM is given in figure: (sketch is schematic and not to scale) 

Which of the following statements is/are true for this motion? 

(A) The force is zero at $t=\frac{3T}{4}$
(B) The magnitude of acceleration is maximum at $t=T$
(C) The speed is maximum at $t=\frac{T}{4}$
(D) The $P.E.$ is equal to $K.E.$ of the oscillation at $t=\frac{T}{2}$

When a body is in $SHM$, then match the following.

One end of a spring of force constant $k$ is fixed to a vertical wall and the other to a block of mass $m$ resting on a smooth horizontal surface. There is another wall at a distance $x_0$, from the block. The spring is then compressed by $2x_0$ and released. The time taken by the block to strike the other wall is