# Illustration Hooke's law - displacement of the spring + restoring force

Hooke's law is illustrated by an elastic spring to which a mass is attached.The spring is deflected from its rest position by the distance $$y$$. Since the spring is stretched, a restoring force $$F$$ is exerted on the mass, acting against the displacement.
The restoring force is linear, which means that if you double the displacement $$y$$, then the force $$F$$ on the mass also doubles:$F ~=~ -D \, y$Here $$D$$ is a spring constant characterizing elasticity of the spring.