A spring is attached to the bottom of an empty swimming pool, with the axis of the spring oriented vertically. An 8 kg block of wood (ρ = 840 kg/m3) is fixed to the top of the spring and compresses it. Then the pool is filled with water, completely covering the block. The spring is now observed to be stretched twice as much as it had been compressed . Determine the percentage of the block's total volume that is hollow. Ignore any air in the hollow space.
