A railway buffer contains a heavy-duty steel spring. During a low-speed coupling test, a compressive force of 4.8 kN4.8\text{ kN}4.8 kN is applied to the buffer, compressing the spring by 15 mm within its elastic limit.
What is the elastic potential energy stored in the spring?
0.036 J0.036\text{ J}0.036 J
72.0 J72.0\text{ J}72.0 J
36.0 J36.0\text{ J}36.0 J
72.0 kJ72.0\text{ kJ}72.0 kJ
