What force must be applied to a spring with a stiffness of 40 N / m to stretch it by 5 cm?

We have a spring stiffness k = 40 N / m, and an extension x = 5 cm.
Task: find the applied force F.
To solve this problem, we use Hooke’s law, the numerical formulation of which is F = kx.
Solution: we translate x = 5 cm into the SI system, where the unit of measurement is not cm, but m (meter).
Since 1 m is 100 cm, then 5 cm = 0.05 m.
We consider the force: F = 40 * 0.05 = 2 N.