Question

A force of $49\, N$ is just able to move a block of wood weighing $10\, kg$ on a rough horizontal surface. Its coefficient of friction is

• A. $ 1 $
• B. 0.7
• C. 0.5
• D. zero

Answer 1

Answer: (C)

When the applied force $F$ is increased the force of static friction $\left(f_{s}\right)$ also increases, but after a certain limit $f_{s}$ cannot increase any more. At this moment the block is just to move
$\therefore F=f_{s} $
and $ f_{s}=\mu R$
where $R$ is the reaction of the surface on the block.
$\therefore F =\mu R$
$\Rightarrow \mu=\frac{F}{R}=\frac{F}{m g}$
Given, $ F=49\, N , m =10 \,kg , g=10\, m / s ^{2} $
$\therefore \mu=\frac{49}{10 \times 10} =0.49 \approx 0.5$