Sunday, September 9, 2012

Breaking Bridges

Whether you are studying Ancient Rome in History, or you are doing an engineering unit in Science, the fact that arches simply carry more weight is an important one for kids to understand.  You might start with the basics and simply build an arch to begin with.  However, the critical objective is that the kids see the difference in strength between two bridges (one that uses a simple beam and one that uses an arch).  Here is a relevant and quick clip from Engineering the Impossible.

Begin with a demonstration by using two heavy objects (the buckets of clay in the photo above) and stretch just a couple of tagboard strips across the expanse, make sure your kids see that the paper sags.  You could put something lightweight on the paper to show how weak the bridge is.  Now, set the heavy objects on a just another sentence strip or two.  Bend the paper strips into an arch with the heavy objects holding the arch in place.  Make sure the apex of the arch is at about the same height as the top of the heavy objects.  Stretch a sentence strip (or other long strip of tagboard) across the expanse again so that your upper strips rest on the lower strips at the apex of the arch.  Put a clip (a paper clip will work) to connect the upper and lower strips and then progressively place heavier and heavier objects on your bridge until it can no-longer hold the weight.

Especially for K-2, you could let them experiment with paper strips and clips on their own at this point, or as an extension, give your kids a bunch of popsicle sticks, old plastic egg halves, clay, glue, string, cardboard, old toilet paper and paper towel rolls, spools and whatever other etcs. you can come up with.  Give them an expanse to cross (12-14 inches works well) with their bridges and let them build a bridge of their own.  Consider the age of your kids and your objectives for the lesson to create a rubric if you plan on assessing the bridges formally and then give them the time to build and experiment.  For older kids, it is common to have a day where you test the strength of the bridges built to the point of collapse.  This can make for a really fun day, but younger kids might have a hard time watching their creations broken so you may want to leave it as a craft for this younger group.


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