Question

In the given figure forces on blocks and coefficient of friction is depicted. Then from following information $[g=10m/s^2]$

• A. Acceleration of 4 kg and 2 kg are different
• B. Acceleration of 4 kg and 2 kg are same
• C. Acceleration of 4 kg is zero
• D. Acceleration of 2 kg is $\frac{1}{6}m/s^2$

Answer 1

Answer: (B), (D)

The correct options are
B Acceleration of 4 kg and 2 kg are same
D Acceleration of 2 kg is $\frac{1}{6}m/s^2$



Assume the two blocks move together. So, the combination of two blocks can be made a system.
The net force on the system is therefore, 25 N. The maximum friction from the bottom surface is $N\mu =60\times 0.4=24N$.
The acceleration of the system = $\frac{25-24}{6}=\frac{1}{6}m/s^2$.
The net force on the 2 kg block is, therefore, $2\times \frac{1}{6}=\frac{1}{3}N$. So, the friction at the top surface is $10-\frac{1}{3}=\frac{29}{3}N$.
The maximum possible friction on the top surface is $N\mu =20\times 0.5=10N$, which is above the calculated value of friction. So, the assumption that the bloks move together is true.