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The course examples connect material performance index values to materials selection charts; the shaft and flywheel examples identify carbon-fibre-reinforced polymers among candidates with high specific strength. Their numerical properties concern specified examples and should not be treated as universal beam compression allowables.

Material selection chooses materials by matching their properties and processing routes to a component's requirements. First exclude candidates violating constraints such as operating temperature, corrosion resistance or minimum stiffness, then compare a performance objective such as mass or cost. A tension member carrying force $F$ over length $L$ needs area at least $F/{\sigma }_{\mathrm{allow}}$, giving mass at least $FL\rho /{\sigma }_{\mathrm{allow}}$. Thus specific strength is relevant for this strength-limited case. A stiffness-limited beam can instead have a different material performance index, so there is no universally best lightweight material independent of geometry and loading.

Specific strength is strength divided by density, with the strength convention stated explicitly. For a uniform tie carrying tensile force $F$, allowable stress ${\sigma }_f$ requires area $A\ge F/{\sigma }_f$. Its mass $m=\rho AL$ is therefore at least $FL/({\sigma }_f/\rho )$. Maximizing ${\sigma }_f/\rho$ minimizes mass under this simple strength-limited model. A material with higher strength can still be worse if its density increases more. Mechanical fatigue, buckling, joining and temperature limits can impose additional constraints not captured by this single ratio.