Question

A force of $49N$ is just able to move a block of wood weighing $10kg$ 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.
$\therefore F=\mu R$
$\Rightarrow \mu =\frac{F}{R}=\frac{F}{mg}$
Given, $F=49N,m=10kg,g=10m/s^2$
$\therefore \mu =\frac{49}{10\times 10}=0.49\approx 0.5$