At first, the center of gravity of wood was not on the ground, and its height from the ground was half the length of the side.
According to the definition of density
The quality of available wood is
m =ρV = 0.8× 10 3×2×0.2×0.2kg = 64kg。
When the wood is placed horizontally, the height of the center of gravity is about 0.1m.
When placed upright, the height of the center of gravity is about1m. ..
The height at which the center of gravity rises from horizontal to vertical is h = (1-0.1) m = 0.9m. 。
In the process of changing wood from horizontal to vertical,
At least the work done by overcoming gravity is equal to the increased gravitational potential energy of wood,
That is w = mgh = 64× 10× 0.9j = 576j.