iii) A conscrvative force F = (4r - 18)i N, where x is in meters, acts on a particle moving along an r axis. The potential encergy U associated with this force is assigned a value of 22 J at x = 0. (a) Write an expression for U as a function of x, with U in joules and x in meters. (b) What is the maximum positive potential energy? At what (c) negative value and (d) positive value of x is the potential encrgy equal to zero? [1+1+1+1]