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(continued) How much weight does your
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:
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
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.