A spring has an unstretched length of $0.30\,\text{m}$ and a spring constant of $400\,\text{N m}^{-1}$. An object is hung from the spring, and the spring is stretched within its limit of proportionality. Its length then becomes $0.50\,\text{m}$. What is the elastic potential energy stored in the spring?
- A$8.0\,\text{J}$
- B$16\,\text{J}$
- C$40\,\text{J}$
- D$50\,\text{J}$