Hooke's Law

A stretched spring pulls back toward its natural length; a squeezed one pushes back. Hooke's law:

$$\relax F = k x$$

$x$ is the stretch or squeeze in metres, $k$ the spring constant in N/m.

The spring force always points back toward the natural length.

Stretched by x, the spring pulls the block back toward x = 0.

A spring holding a mass at rest: $\relax kx = mg$.