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Answer» Let
f1 = frictional force between 15kg BLOCK and ground surface
f2 = frictional force between 5kg block and 15kg block surface
N1 = normal reaction on block 15kg block
N2 = normal reaction on block 5kg block
Because there is no upward motion
N1 = 15g + 5g = 20G
N2 = 5g
F (limiting) = µN1 = 0.6×20g = 0.6×20×10 = 120N
Because Ground can friction force can able to oppose 120N if we apply force above 120N then only block starts to MOVE . In this case block isn't move .
f1= F
f1 = 80N(because it is self ADJUSTING force)
because there is no actual relative motion and also there is no impendig of relative motion(tendency of relative motion) HENCE there is no force acting surfaces between 5kg block and 15kg block
f2 = 0 N
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