If the work required to stretch a spring 4 feet beyond its natural length is 18 foot-pounds, how much work is needed to stretch it 20 inches beyond its natural length? Your answer must include the correct units. (You may enter lbf or lb*ft for ft-lb.)

work done by stretching a spring by x m

= potential energy of stretched spring = 1/2 k x² where k is spring constant and x is extension.

18 = 1/2 k x 4² (Given)

k = 2.25 poundal / ft

work needed to stretch 20 inch or 20/12 ft

= 1/2 x k (20/12) ²

= 1/2 x 2.25 x 400/144

= 3.125 ft-poundal.