Hi: 1. Why is more work needed to stretch a spring by 1cm from its unextended length, than is required to stretch it by an extra 1cm when the spring is already extended? Answer Up until the elastic limit the spring obeys Hooke's law and the force applied is equivalent to the extension. After the elastic limit a greater force is required to stretch the spring for the same length as would be possible before the elastic limit. What actually goes on inside the spring to make this true is anybody's guess. Hazard a try?