Question

A cubic block of mass $ m $ is sliding down on an inclined plane at $ 60^\circ $ with an acceleration of $ \frac{g}{2} $, the value of coefficient of kinetic friction is:

• A. \( \sqrt{3} - 1 \)
• B. \( \frac{\sqrt{3}}{2} \)
• C. \( \frac{\sqrt{2}}{3} \)
• D. \( 1 - \frac{\sqrt{3}}{2} \)

Hint:

The coefficient of kinetic friction can be found by analyzing the forces acting on the object and using the equation for the net force along the incline.

Answer 1

The Correct Option is A

Given: 
- The angle of inclination, \( \theta = 60^\circ \), 
- The acceleration of the block, \( a = \frac{g}{2} \), 
- The gravitational acceleration, \( g \). We need to find the coefficient of kinetic friction, \( \mu_k \). 
Step 1: Analyze the forces acting on the block.
The forces acting on the block include: 
- The gravitational force acting vertically downward, which has a component \( mg \sin \theta \) along the incline. 
- The normal force, \( N = mg \cos \theta \). 
- The frictional force opposing the motion, \( F_f = \mu_k N = \mu_k mg \cos \theta \). The net force causing the block to slide down is: \[ F_{\text{net}} = mg \sin \theta - F_f \] The net force is also equal to the mass times the acceleration: \[ F_{\text{net}} = ma \] 
Step 2: Set up the equation.
Equating the two expressions for \( F_{\text{net}} \): \[ mg \sin \theta - \mu_k mg \cos \theta = ma \] Since the acceleration \( a = \frac{g}{2} \), substitute this into the equation: \[ mg \sin \theta - \mu_k mg \cos \theta = m \cdot \frac{g}{2} \] Canceling out the mass \( m \) on both sides: \[ g \sin \theta - \mu_k g \cos \theta = \frac{g}{2} \] 
Step 3: Simplify the equation.
Substitute \( \theta = 60^\circ \) into the equation: \[ g \sin 60^\circ - \mu_k g \cos 60^\circ = \frac{g}{2} \] Using the known values \( \sin 60^\circ = \frac{\sqrt{3}}{2} \) and \( \cos 60^\circ = \frac{1}{2} \): \[ g \cdot \frac{\sqrt{3}}{2} - \mu_k g \cdot \frac{1}{2} = \frac{g}{2} \] Dividing through by \( g \) and simplifying: \[ \frac{\sqrt{3}}{2} - \frac{\mu_k}{2} = \frac{1}{2} \] Multiplying through by 2: \[ \sqrt{3} - \mu_k = 1 \] 
Step 4: Solve for \( \mu_k \).
\[ \mu_k = \sqrt{3} - 1 \] 
Final Answer \sqrt{3 - 1}