(i)-[C], (ii)-[C], (iii)-[B], [C], (iv)-[A], [B], [C]

Sol. Let f 1 , f 2 and f 3 represent the friction forces between three contact surface A – B, B – C and C – ground respectively. Limiting values of friction force at three surface are 8N, 15 N and 10N respectively.

For relative motion between C and ground, the minimum force needed is f = 10 N.

For F = 12 N

All the three blocks move together with same acceleration

i.e. a 1 = a 2 = a 3 = a

F – f 3 = (2 + 3 + 5) a

∴ a = = m/s 2 f 1 = 2a = N

f 2 = 12 – f 1 – 3a = 11N

f 3 = 10 N

For F = 15 N: The situation is similar.

For relative motion to start between B and C,
f 2 ≥ f 12

F – f 3 = 10 a and dF – f 2 = 5a

or f 2 = F – 5a = F – 5 =

or > 15 or F > 20 N

[condition for relative motion to start between B and C].

For relative motion to start between A and B.

f 1 ≥ = 8 N

F – f 1 – f 2 = 3a and f 1 = 2a

f 1 = 2 > 8

or f > 35 N [condition for relative motion between A and B].