Question

Find the value of friction force if the system is in equilibrium.

• A. $10\text{N}$
• B. $30\text{N}$
• C. $20\text{N}$
• D. $15\text{N}$

Answer 1

The correct option is C $20\text{N}$

FBD for the block kept on the surface

Normal force $N=10g-4g\sin {60}^{\circ }=100-40\frac{\sqrt{3}}{2}$
$N=100-34.64=65.35N$
Friction force $f_{max}=\mu N=0.4\times 65.35=26.14N$

Horizontal component is only responsible for moving the block so,
$f=4g\cos {60}^{\circ }=40\times \frac{1}{2}=20N$