Stretch a spring and it answers in proportion: $F = kx$ — F equals k x. The spring constant $k$ is the spring's price list, in newtons per metre: how many newtons each metre of stretch will cost you. Stiff springs charge hundreds; a ballpoint's spring charges pocket change. Double the stretch, double the force — that straight-line honesty is the whole law.

The trap this lesson keeps setting: springs are small, so problems hand you centimetres, while $k$ thinks in metres. Convert BEFORE the sockets — $5\ \mathrm{cm} = 0.05\ \mathrm{m}$, centi means a hundredth — or your force arrives a hundred times too proud.