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Discussing Results (continued)

Before testing, or during the first test, discuss what constitutes a fair test by asking how and where the weights should be placed on the bridges. Should the weight be dropped onto the bridge? Placed gently onto the bridge? If your class decides that all weight must be placed gently in the center, they are recognizing that the weight, its position, and how it is placed on the bridge may all be relevant variables in this experiment. In order to get "fair" objective results, only one variable may change (the weight), and the others (placement and position of the weight) must remain unchanged. The idea of a fair test is fundamental to the concept of scientific objectivity.

As you begin the adding of weights, the students themselves will generate many questions for discussion. They will be interested in which bridge holds the most weight, and exactly how much it holds.

Keep adding the weights to the bridge until it breaks. Questions connected with the breaking can include the following:

• How much weight does your bridge hold?

• Where does the bridge start to break? What do you think is happening?

• Why do you think one bridge is stronger than another?

• How could a bridge be made stronger?

How could you build a strong bridge using less newspaper? (This question can lead to a possible follow up activity.)

Building bridges demonstrates the effect of weight or another force at a distance from a pivot or a support point (torque). In addition, it makes clear the need for the upward support of the bridge to counteract the downward force of the load.

One of the clearest principles illustrated in this activity has to do with beams. If the span of the newspaper bridge is considered as a beam, the crushing and bending of the bridge under its load make the stresses on the beam quite evident.