A balsa bridge classroom lesson plan

Short answer: five to six sessions work well: introduce forces, design and plan, build, test, analyze and redesign. Students learn the most from breaking a bridge and explaining why.

Goals

Students should be able to:

• Describe tension and compression and find them in a truss.
• Explain why triangles make a frame stiff.
• Calculate a load-to-mass ratio.
• Use test results to improve a design.

Materials per team

Balsa sticks, a hobby knife or craft cutters, a cutting mat, sandpaper on a block, wood glue, pins, a board, wax paper, a scale, a bucket and weights or sand, and safety glasses.

Outline

1. Forces and trusses. Push and pull a stick, see it bow. Show tension and compression. See truss basics.
2. Design. Teams sketch a bridge to a span and mass limit, and mark where they think each stick is pushed or pulled. See which members are pushed or pulled.
3. Build. Teams cut and glue on a plan. See step-by-step building.
4. Test. Add load gradually, record, and photograph the break. See testing safely.
5. Analyze. Calculate efficiency, and explain what failed and why. See efficiency.
6. Redesign (optional). Rebuild with one change and compare.

Assessment ideas

A design sketch with predictions, a build log, a test record, and a short written or spoken explanation of the first failure and one improvement.

Classroom safety

Safety glasses at every test, sharp blades handled by one student at a time, and a clear floor under the load. Keep a sand-filled bucket at a safe height.