Question

Two masses A and B of $30$ kg and $10$ kg are connected with a string passing over a frictionless pulley fixed at the corner of a table, as shown in figure. The coefficient of friction between the table and block is $0.2$. What is the minimum mass of C that may be placed on A to prevent it from moving?

Answer 1

Given mass A=30 kg and mass B=10 kg and coefficient of friction $\mu$=0.2

Thus tension in the sting in which mass B is hanging $T=m_{B}g$

$T=100N$ here g is considered 10 m/$s^2$

Now for the system A+C, tension $T=\mu N$, where N is the normal force.

$100=0.2(30+m_c)10$

$50-30=m_c$

$m_c=20kg$