Remember how a spring works: when you pull on it$,$ it stretches. Newton's $3^{rd}$ Law of motion says that the spring pulls back on you too. If you pull the end of the spring in the $+x$ direction with a force, vec$(F),$ in the $+x$ direction, the spring stretches by an amount $x.$ You know that the force has only an $x$ component, and hence

$F_x=kx$

where $F_x$ is the force applied, $k$ is the force constant of the spring, and $x$ is how much the spring stretches.

$14-2$