Question

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 weight of blocks are A = 300 N, B = 100 N, C = 200 N. Between A and B, µ = 0.3. Between B and C, µ = 0.2. Between C and the ground µ = 0.1.

(A) 90 N 

(B) 60 N 

(C) 80 N 

(D) 100 N

Answer 1

Correct option is (B) 60 N

Now let us check the limiting frictions between the three surfaces

∴ Now let us assume P would be greater than 60 N and less than 80 N.

For this P;

∴Here f₂ is greater than the maximum static friction between C and ground. Hence the block C will slide on the ground. There by all the three blocks will slide for a minimum force of 60 N.

Answer 2

Correct answer is (B) 60 N