A collar B of weight W can move freely along the vertical rod shown. The constant of the spring is k, and the spring is unstretched when θ = 0.
(a) Derive an equation in θ, W, k, and l that must be satisfied when the collar is in equilibrium.
(b) Knowing that W = 300 N, l = 500 mm, and k = 800 N/m, determine the value of u corresponding to equilibrium.