Testing Assumptions
The model structures short lab is all about testing assumptions, and seeing how the assumptions made
about any particular situation will affect the predictions.
A simple home experiment to test model assumptions
Take a steel ruler and measure its dimensions to calculate its value of I. Now form a cantilever by
holding one end on the edge of a bench, leaving all but 2 or 3cm sticking out. Place a pound coin on
the end of the ruler and estimate the resulting deflection. How does it compare to the value given by
the databook? (PL3/3EI).
Most people find that the deflection is greater than predicted. What assumptions have you made that
may not be true? Think about the support - is it rigid? Or does your hand allow the ruler to rotate a
little, increasing the tip deflection? What about if you hold it with a single finger? What about if
you push down very hard, or would this also deflect the bench and give another rotation?
When you carry out the structures lab, think about how the apparatus relates to simplified models.
Don't forget that instrumentation may have its own limitations.
© Cambridge University, Engineering Department
Last updated 12/10/2012 by hrs@eng