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Breaking Bridges Rating: 4

 Level: Beginner Duration: 2 - 4 class periods Grade Level: NA Author: Pitsco Topics: Stress, strain, load, tension, compression, torsion, geometric stability, truss design, calculating efficiencies, modeling techniques Materials:  Toothpick Bridges – Getting Started Package, foam board, weight (pennies or sand), paper to make class data table, graphing paper .

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Objective

Determine the breaking point of a toothpick bridge.

Description

Pitsco’s Toothpick Bridges activity encourages the design process in the construction of a bridge from ordinary toothpicks and glue. Students see the strength of triangle shapes, trusses, and glue joints as they build their bridges. The bridges are destructively tested by adding loads to the middle of the bridges.

Procedures

Students formulate a bridge design based on the length of one toothpick. Typically, the bridges will need to span eight to 12 inches, and students design one side of the bridge to accommodate the span. The bridge design is attached to a piece of foam board, covered with waxed paper, and, one by one, toothpicks are overlaid above the drawing on the waxed paper and glued together.

After one side of the bridge is complete, the glue is allowed to dry, and then the bridge side is carefully removed from the waxed paper. A second bridge side is constructed in similar fashion and allowed to dry.

The two bridge sides are assembled parallel to each other, a little less than a toothpick length apart. Toothpicks are added and glued between the two bridge sides. The completed bridge is allowed to dry overnight, and then it is tested by attaching a bucket to the bridge center and adding pennies or sand until the bridge breaks. A data sheet for the class can be made including the mass of the bridge, the mass the bridge held, and the efficiency of the bridge.

Discussion

Compare results of other students’ bridges – construction, design, and breaking point.

Continuation

From the data collected, the class average for the mass held and the class average for efficiency can be calculated: Efficiency = (Mass of Bridge/Mass of Load).

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