A spring with spring constant k = 800 N/m is compressed 12 cm from its equilibrium position. A spring with spring constant k = 400 N/m has the same elastic potential energy as the first spring when its extension is
1) 0.060 cm.
2) 0.085 cm.
3) 0.12 cm.
4) 0.17 cm.
5) 0.24 cm.
Answer: 4