For a spring, both the magnitude and direction of the spring force vary as the spring is stretched or compressed.

The force exerted by ideal spring is given by Hooke's Law:  F = -k x

k: the spring constant, a stiffness measure (units N/m).

Minus sign: spring force opposite in direction to the displacement.

x: displacement from the equilibrium position of the spring.

What is the spring constant of our simulated spring?

The spring constant is:    Force/extension =