In the diagram shown, the blocks A, B and C weight, 3kg, 4kg and 5kg respectively. The coefficient of sliding friction between any two surface is 0.25. A is held at rest by a massless rigid rod fixed to the wall while B and C are connected by a light flexible cord passing around a frictionless pulley. Find the force F necessary to drag C along the horizontal surface to the left at constant speed. Assume that the arrangement shown in the diagram, B on C and A on B, is maintained all through. `(g=9.8m//s^2)`

Answer

A

Explanation

When force F is applied on C, the block C will move towards left.

F.B.D. for mass C is

As C is moving with constant speed `F=f_1+f_2+T` …(i)
F.B.D. for mass B is

As B is moving with constant speed `f_2+f_3=T` (ii)
Subtracting (ii) from (i) `F-(f_2+f_3)=f_1+f_2+T-T=f_1+f_2`
`implies F=f_1+2f_2+f_3=mu(m_A+m_B+m_C)g+2mu(m_A+m_B)+mum_Ag`
`F=mu(4m_A+3m_B+m_C)g`
`=0.25[4xx3+3xx4+5]xx9.8=71.05N`