This investigation is effectively testing the internal forces acting within the material used to make the weight-bearing surface of the bridge. The material remains the same for each bridge and the length of the material used is the independent variable. This is a simple investigation to conduct and the variables are easy to control. It is ideal for pupils to do as a completely independent investigation and could therefore be used as an assessment piece for their understanding of fair testing/ investigation structures/ data handling/ drawing of conclusions from data.
Here is an interactive simulation to help reinforce the learning objectives: