A wooden block of mass 0.5 kg and density 800 `kgm^-3` is fastened to the free end of a vetical spring of spring cosntant 50 N`m^-1` fixed at the bottom. If the entire system is completely immersed in water, find a. the elongation (or compresion) of the spring in equilibrium and b. the time period of vertical oscillations of het block when it is slightly depressed and released.