Find the least horizontal force P to start motion of any part of the system of the three blocks resting upon one another as shown in figure The weights of blocks are `A = 300 N , B = 100 N` and `C = 200 N` .Between A and B , the coefficient of friction is `0.3` between B and C is `0.2` and between C and the ground is `0.1`

Options

A. `60 N`
B. `90 N`
C. `80 N`
D. `70 N`

Answer

a

Explanation

`f_(l_1) = mu_(1)m_(A)g = 0.3 xx 300 = 90 N`
`f_(l_2) = mu_(2)(m_(A) +m_(B))g`
` = 0.2(300 + 100) = 80 N`
`f_(l_3) = mu_(3)m_(A) + m_(B) +m_(c)g`
`= 0.1(300 + 100 +200)= 60 N`