Physics - NLM Question with Solution | TestHub

Friction between block & plank $\mathrm{f}=0.4\times 50=20\mathrm{N}$.

Acceleration of block ${\mathrm{a}}_{\mathrm{b}}=-4\mathrm{m}/{\mathrm{s}}^2$

Acceleration of block ${\mathrm{a}}_{\mathrm{p}}=2\mathrm{m}/{\mathrm{s}}^2$ ]

${\mathrm{a}}_{\mathrm{b}/\mathrm{p}}=-6\mathrm{m}/{\mathrm{s}}^2$

w.r.t plank

$\mathrm{v}=\mathrm{u}+\mathrm{at}$

$0=18-6\mathrm{t}\mathrm{t}=3\sec$

Disp of block w.r.t. plank

$\mathrm{s}=4\mathrm{t}+\frac{1}{2}{\mathrm{at}}^2=18\times 3-\frac{1}{2}\times 6\times 3^2$

$=54-27=27\mathrm{m}$

minimum length of plank 27 m