Question

A block of mass $0.1\text{kg}$ is held against a wall by applying a horizontal force of $5\text{N}$ on the block. If the coefficient of friction between the block and the wall is $0.5$, the magnitude of the frictional force acting on the block is
[Take $g=9.8{\text{m/s}}^2$]

• A. $2.5\text{N}$
• B. $0.98\text{N}$
• C. $4.9\text{N}$
• D. $0.49\text{N}$

Answer 1

The correct option is B $0.98\text{N}$

Here, gravitational force $\left(mg\right)$ is the only external force acting on the block.

$\therefore F=mg=0.1\times 9.8=0.98\text{N}$

And, maximum friction force that can act on the block is

$f_{max}=\mu N=0.5\times 5=2.5\text{N}$.

Here, $F<f_{max}$, and the block is held against a wall, so the magnitude of friction force $f$ will be equal to the weight of the block.

$\therefore f=F=0.98\text{N}$

Maximum friction force that can act on block is $f_{max}=\mu N=0.5\times 5=2.5\text{N}$.
Required force to hold the block is less than the $f_{max}$. So, $0.98\text{N}$ of force will be acting on the block.

So, option $\left(b\right)$ is correct.

Why this question? Caution: Maximum friction force that can act on block is $2.5\text{N}$. But this is not the friction force that is acting on the body.