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How Bags Work

Shopping bags carry their loads by resisting forces that tend to stress the bag's material until it fails. Getting practice at “seeing the stresses” in a loaded bag can help your students get better at predicting where bag failure will likely occur, and thus help them prevent it.

In a standard shopping bag filled with potatoes (see Figure 1), they reveal where the material is experiencing force. Lots of wrinkles mean higher stresses. (See the edges of the handle). Other areas, like the sides, are slack because the material experiences little stress. Much of the potatoes' weight is being carried by a small section of material near the handle cutout.

 

Stress is the intensity of a force that structural material must bear. It is expressed in pounds of force per square inch. Stress can act in different directions: pulling apart (tensile); pushing together (compressive); and shear (forces that slide adjacent regions past one another, like a deck of cards).

Bag failure begins at places where (a) the stress is constant and exceeds the level that the material can sustain (b) or in places where the bag is weaker (e.g., place where an attached handle is welded or glued) failure is likely there. What else can cause stress to be higher in some places more than others? To understand this, you need to look for concentration of forces and at how the bag's structure transmits its load forces to the handles.

But when the force might be carried anywhere within the whole width of the side of the bag, how do you decide where failure is likely to occur?

Is it spread out uniformly or focused in one part? How broad or narrow are the materials that are transmitting the force?

To help understand how concentration of forces causes failure, imagine a set of strings that connect to a single string and are hoisting a platform with weights. The four strings are tied off onto a ring -- the strings and ring are supported by a single string. If all strings are made of the same material, which string would first fail? Why?

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