Hooke's Law states that the force F required to compress or stretch a spring (within its elastic limits) is proportional to the distance d that the spring is compressed or stretched from its original length. That is, F = kd, where k is the measure of the stiffness of the spring and is called the spring constant. The table shows the elongation din centimeters of a spring when a force of F kilograms is applied. Force, F Elongation, d 20 40 60 80