The triangular frame shown in Figure 8 is made up of three uniform slender rods, each of length a = 0.3 m and a mass m = 0.6 kg. It is pivoted about a horizontal axis through the vertex A and attached to two springs, each of stiffness k = 100 N/m, at right angles to AB and BC respectively. A block D is connected to the spring at C and is allowed to have a harmonic displacement given by sD = b sin ωt (t being time in s and b = 2 mm), in a groove as shown in the figure. Determine (a) the differential equation of small oscillations of the frame in the vertical plane and (b) the critical driving frequency ωc of the block D which will cause the system to have excessively large oscillations.