Question

A spring 20cm long is stretched to 25cm by a load of 50N. What will be it's length when stretched by 100N assuming that the elastic limit is not reached

Answer 1

The Hooke's law says

$$F=k\Delta l$$

Hence

$$l_2=\frac{F_2}{F_1}(l_1-l_0)+l_0\\ =100/50*(25-20)+20=30\:\rm cm$$