$($b$)$ The mass m$=1$ kg is attached to the rigid stem pivoting at O and is attached to two

springs k and $3$k $($with k$=20$ N$/$m$),$ as shown in figure QB$1$b$.$ The distances a and

b are a$=0\mathrm{.}5$m and b$=1$m$.$ Using the moment equation and the small angle

approximation, find wn the natural angular frequency of this system. What would

be the minimum value of k to have a vibration $($i$.$e$.$ harmonic motion$)?$