We have six identical wooden shelves with a length of 50 cm, a width of 10 cm and a height of 4 cm. All shelves initially lie on the floor, on the largest surface. The density of the wood from which the shelves are made is 
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The six shelves are stacked on top of each other so that they all lie on the largest surface. Calculate the change in potential energy for the top shelf.
Calculate the change in potential energy for all six shelves together. Note that at the beginning of the observation all the shelves lie on the floor, and at the end they are stacked on top of each other.
