Two equal masses are attached to the two ends of a spring of spring constant k. The masses are pulled apart symmetrically to stretch the spring by a length x over its natural length. The work done by the spring on each mass is:
−14kx2
Total work done by spring is −12kx2.
Here (- ve) sign because force and displacement are in opposite direction.
Work done on both the blocks is same.
So, work done on each block is −14kx2