Rough things!
Hold nine books horizontally between your hands, as shown above. The coefficients of friction between any two surfaces in contact are all the same (including that between a hand and a book). If you slowly loosen the grip, which book(s) will tend to slide down first?
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1 solution
If we consider a bunch of the books, let's say from 2-6, then the friction force acting on the outer surfaces of the outermost books (that is 2 and 6), will carry the weight of bunch.
2 f 2 6 = W 2 6
The bigger is the bunch, the greater is the requirement of the frictional force. Hence, the hands need to apply the greatest friction force on the books. The normal reaction and the coefficient of friction between all the surfaces are equal. Therefore, when the grip is loosened, then the whole bunch of the books will slip from the hands.