A college student constructs a book case using planks of wood placed between stacks of bricks and fills each shelf completely with books. The wood shelves sag but do not break. Four years later, when the student graduates, and moves her books the book shelves remain bent and she has to throw them away. Explain the physics of what happened to the planks of wood in terms of stretching, tension, and compression?
When the student put the books on the shelves, the weight load caused the shelves to bow, that put the top of the shelves in compression and the bottom of the shelves in tension. The compression and tension were still within the tolerance of the wood's strengths, as the wood didn't break. However, the wood became permanently disfigured as the top of the shelves compressed and the bottom of the shelves stretched, trying to maintain equilibrium. That is a common problem in not only bookcases but other structures made of wood.