Physics - Kinematics Question with Solution | TestHub

Velocity of train A
${\mathrm{V}}_{\mathrm{A}}=90\frac{\mathrm{km}}{\mathrm{hr}}=90\times \frac{5}{18}=25\mathrm{m}/\mathrm{s}$
Velocity of train B
${\mathrm{V}}_{\mathrm{B}}=54\frac{\mathrm{km}}{\mathrm{hr}}=54\times \frac{5}{18}=15\mathrm{m}/\mathrm{s}$
Velocity of train B w.r.t. train
$\begin{array}{rl}A={\vec{V}}_B& -{\vec{V}}_A\\ & =15-(-25)\mathrm{m}/\mathrm{s}\\ & =40\mathrm{m}/\mathrm{s}\end{array}$
Time of crossing $=\frac{\text{ length of train }}{\text{ relative velocity }}$
$(8)=\frac{\ell }{40}$
$\ell =8\times 40=320$ meter.