# If the length of a 47 cm spring increases to 89 cm when a 2 kg weight is hanging from it, what is the spring's constant?

We should work in SI units, so the stretch is $0.89 - 0.47 = 0.42$ $m$. Then we know the weight force of a $2$ $k g$ mass is $19.6$ $N$ and $F = k x$, and can rearrange to give: $k = \frac{F}{x} = \frac{19.6}{0.42} = 46.7$ $N {m}^{-} 1 \mathmr{and} k g {s}^{-} 2$.