This is the law that shows the relationship between the extension (or  elongation) of a spring and the force that is applied to it. It is applicable when the spring has not reached its elastic limit. Once the elastic limit has been exceeded, the extension of the spring is no longer directly proportional to the force applied.

 
If a graph of F against x is plotted, the gradient of the linear line is k. The unit for k is N/m² or N/cm². 
 
When 2 springs are connected in parallel, the spring constant for the system (of 2 parallel springs) is 2k. If 3 springs are fixed in parallel then the spring constant of the system of springs is 3k, and so on.
 
When 2 springs are connected in series, the spring constant of the system of springs is k/2. With 3 springs, it is k/3 and so on.
 
The bigger the spring constant, the stiffer (the less elastic) is the spring.

Hukum ini menunjukkan perhubungan di antara pemanjangan spring dan daya yang dikenakan ke atas spring. Hukum ini benar sekiranya had elastik spring belum dilampaui. Setelah had elastik dilampaui, pemanjangan tidak lagi berkadar terus dengan daya yang dikenakan.