Question 7.3: A cantilever of length 2 metre fails when a load of 2 kN is ...

Given :
Length,   L = 2 m = 2 × 10³ mm
Load,       W = 2 kN = 2000 N
Section of beam is 40 mm × 60 mm.
∴ Width of beam, b = 40 mm
Depth of beam,    d = 60 mm

Fig. 7.10 (a) shows the section of the beam.
Section modulus of a rectangular section is given by equation (7.7).

∴                      Z=\frac{b d^2}{6}=\frac{40 \times 60^2}{6}=24000  mm ^3

Maximum bending moment for a cantilever shown in Fig. 7.10 is at the fixed end.

∴              M=W \times L=2000 \times 2 \times 10^3=4 \times 10^6  Nmm

Let           σ_{\max} = Stress at the failure

Using equation (7.6), we get

M=\sigma_{\max } . Z

∴                \sigma_{\max }=\frac{M}{Z}=\frac{4 \times 10^6}{24000}= \pmb{1 6 6 . 6 7  N / m m ^2.}