A beam calculation mixes metre and millimetre units
A fictional classroom beam is simply supported over two metres and carries a central six-kilonewton point load. For this ideal model the maximum bending moment is supplied by FL/4. The rectangular section is forty millimetres wide and eighty millimetres deep, and the supplied section modulus is 42667 cubic millimetres. A student divides the numerical moment in newton metres directly by that section modulus and reports a very small stress. Another converts the moment to newton millimetres before using stress equals moment divided by section modulus. This written exercise approves no structure and asks for a consistent unit calculation rather than a complete real design assessment.
1. What is the model’s maximum moment?
Read the worked answer
6000 N multiplied by 2 m divided by 4 gives 3000 N m, or 3000000 N mm.
2. What is the resulting extreme-fibre stress?
Read the worked answer
3000000 divided by 42667 is approximately 70.3 N/mm², equivalent to 70.3 MPa. Actual design also requires other applicable load, stability and material checks.