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.
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.