Question

In the figure $m_A=m_B=m_C=60$ kg. The coefficient of friction between C and ground is 0.5, B and ground is 0.3, A and B is 0.4. C is pulling the string with the maximum possible force without moving. Then the tension in the string connected to A will be:

• A. 120 N
• B. 60 N
• C. 100 N
• D. zero

Answer 1

The correct option is D zero

Let us consider that maximum force applicable by the man is equal to the maximum static friction possible to act on the man.

Thus $F=\mu mg=0.5mg$

The maximum friction force possible to exist between A and B=$\mu mg=0.4mg$

Maximum friction possible between block B and ground=$\mu N=0.3\times 2mg=0.6mg$

If tension must exist to balance the forces acting on the block B,

$T=F-f_1-f_2=0.5mg-0.6mg-0.4mg<0$

Thus tension in the string would be zero actually, and the frictional forces acting between surfaces would not have the maximum values possible, but lesser than those to balance the force F.