4.9 H {a} The force exerted by a onedimensional spring, xed at one end, is F = k.r, where x is the displacement of the other end from its equilibrium position. Assuming that this force is conservative (which it is) show that the corresponding potential energy is U = ital, if we choose U to be zero at the equilibrium position. {b} Suppose that this spring is hung vertically from the ceiling with a mass or suspended from the other end and constrained to move in the vertical direction only. Find the extension I.) of the new equilibrium position with the suspended mass. Show that the total potential energy (spring Plus gravity} has the same form ilk}; if we use the coordinate y equal to the displacement measured from the new equilibrium position at x = x0 (and redene our reference point so that U = i] at y = U)