A stack of 40 books lies on the ground. Each of them is d = 2 cm thick and has mass m = 1 kg. We want to place the books into 4 shelves, with each shelf containing a stack of 10 books lying on the side and the shelves' are at heights 100 cm, 130 cm, 160 cm and 190 cm. What work do we have to perform to move the books? Mirek was unable to move full bookshelves.