Example

Question:

The two thigh bones (femurs), each of cross-sectional area \(10\,\mathrm{cm}^2\) support the upper part of a human body of mass 40 kg. Estimate the average pressure sustained by the femurs.

Solution:

The total cross-sectional area of the femurs is \[ A = 2 \times 10\,\mathrm{cm}^2 = 20 \times 10^{-4}\,\mathrm{m}^2 \]
The force acting on them is \[ F = 40\,\mathrm{kg} \times 10\,\mathrm{m\,s}^{-2} = 400\,\mathrm{N} \] (taking \(g = 10\,\mathrm{m\,s}^{-2}\)).
The average pressure is \[ P_{\mathrm{av}} = \frac{F}{A} = 2 \times 10^5\,\mathrm{N\,m}^{-2} \]