Figure for Problem 20 21. The spring is unstretched at the position x = 0. Under the action of a force P, the cart moves from the initial position x =-150 mm to the final position x2 = 80 mm. Determine (a) the work done on the cart by the spring and (b) the work done on the cart by its weight. Unstretched position 500 N/m 6 kg 15 Figure for Problem 21