Question

A horizontal force of $10N$ is necessary to just hold a block stationary against a wall. The coefficient of friction between the block and the wall is $0.2$. Find out the weight of the block.

• A. $100N$
• B. $2N$
• C. $20N$
• D. $50N$

Answer 1

The correct option is B $2N$

$\text{Step 1: Drawing FBD}$ $\text{[Refer Figure]}$

$R=$ Normal reaction by wall

$f=$ frictional force between wall and block

$\text{Step 2: Applying Newton's Second Law}$

For block to be stationary against the wall. net force acting on it should be zero.

$\therefore \sum F_x=0$

$\Rightarrow R-10\left(N\right)=0$

$\Rightarrow$ $R=10N$

And

$\sum F_y=0$

$\Rightarrow$ $f-mg=0$

$\Rightarrow$ $f=W$ $....\left(1\right)$

$\text{Step 3: Applying Condition of Friction}$

Since the block is just held stationary, So maximum friction should act

$\therefore f=\mu R=0.2\times 10N$

$=2N$

Using Equation $\left(1\right)$, Weight of the block $W=2N$

Hence option B is correct.