Question

A block of mass $70kg$ is kept on a rough horizontal surface $(\mu =0.4)$. A person is trying to pull the block by applying a horizontal force, but the block is not moving. The net contact force exerted by the surface on the block is $F$, then;

• A. $F=280N$
• B. $F=754N$
  
• C. $700N\le F\le 754N$
• D. $F=700N$

Answer 1

The correct option is C $700N\le F\le 754N$

Draw a free body diagram of the block.

Given, mass of block, m = 70 kg,
To find, frictional force, f

Formula used : $F=\sqrt{N^2+f^2}$
Normal force,$N=70kg\times 10N/m^2=700N$
Frictional force,$f=\mu N=0.4\times 700N=280N$
Net contact force: $F=\sqrt{N^2+f^2}$
$N\le F\le \sqrt{N^2+f^2}$
$700N\le F\le \sqrt{(700N{)}^2+(280N{)}^2}$
$\Rightarrow 700N\le F\le 754N$