A girl holds a book of mass $m$ against a vertical wall with a horizontal force $F$ using her finger, so that the book does not move. The frictional force on the book by the wall is
KVPY 2012, Medium
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(b)
Minimum force $F$ must be such that generated friction is able to balance weight $m g$ of book.
So, $\quad f=\mu N=\mu F$.
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