Hooke's Law states that the amount of stretch or compression in a spring is directly proportional to the applied force.
Use this set of 3 different springs to quantitatively demonstrate this principal first hand. Springs are made of stainless steel with loops on each end. All three springs are of a different size and stiffness. Set consists of one 6mm dia x 55mm in length small spring. One 11mm dia x 74mm in length medium spring. One 14mm dia x 83mm in length large spring.