Question

In the figure shown, blocks $A,B$ and $C$ weigh $3\text{kg},4\text{kg}$ and $8\text{kg}$ respectively. The coefficient of sliding friction between any two surfaces is $0.25$. $A$ is held at rest by a massless rigid rod fixed to the wall while $B$ and $C$ are connected by a string passing round a frictionless pulley. Find the force (in $N$) needed to drag $C$ along the horizontal surface to left at constant speed.

Assume the arrangement shown in figure is maintained all through. $(g=10{\text{m/s}}^2)$

• A. $75$
• B. $80$
• C. $85$
• D. None of these

Answer 1

The correct option is B $80$


From free body diagram of $B$
$T=f_1+f_2=3\mu g+7\mu g=10\mu g$
$=10\times 0.25\times 10=25\text{N}$

From free body diagram of $C$
$F=f_2+f_3+T$
$F=\mu .7g+\mu .15g+10\mu g=80\text{N}$