Stress in Paper

The paper bridge keeps the water bottles from falling by applying an upward force that is equal to the force of gravity acting downward on the bottle.

To understand how stress works in paper, here are two key ideas to know:
1. As in any other material, stress is the concentration of force through an amount of material (see How Bags Work in the Shopping Bag activity). Cut in half the width of a band of paper carrying a given tension load in tension, for example, and the stress on the paper that remains will double.
2. A broad sheet of paper will have stresses only in certain portions. The paper will show you where the stresses are, and what direction the forces go, by forming long wrinkles that line up with the direction of force. (See How Bags Fail in the Shopping Bag portion of DITC).

Two other ideas about paper as a building material that can help to know are:

• Microscopically, paper is a pile of separate fibers loosely bundled and stuck together. If you bend, fold, or spindle it, the fibers start coming unstuck. This significantly weakens the paper. Remember that to control the way that a sheet of paper tears, you crease it before pulling it apart.
• Paper tends to start to tear wherever there’s a notch or sharp inside corner. The force coming past these notches gets crowded together as it dodges around them. Crowded force equals higher stress! Students should try to cut edges in their bridges as smoothly as possible, especially on interior curves.