A force of 18 lb is required to hold a spring stretched 8 in. beyond its natural length. How much work W is done in stretching it from its natural length to 13 in. beyond its natural length? W = ft-Ib Shown is the graph of a force function (in newtons) that increases to its maximum value and then remains constant. F (N) 510152025303540 ~6""'"' How much work Wis done (in J) by the force in moving an object a distance of 40 m? w=:x1