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If we exclude the motion due to the rotation of the Earth, what are the forces acting on the book? I know about the force of mg acting on the book and its opposite acting on the Earth, but I have some confusion relating to the concept of Normal Reaction. If the ground, let's call it B, exerts a Normal reaction N on the book, A, then wouldn't A exert an opposite force to N? And if it does so, why does the book remain at rest and not continue accelerating towards the centre of the Earth?

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The force of gravity is not a reaction to the normal force but vice versa. Gravity acts on the book exerting a force. The book then exerts an equal but opposite force on the earth (so away from the centre) according to Newton's third law.

The book stays at rest because the Earth is 'strong' enough to support the book. If you put a heavy book on a old, weak table, it could break. The table has a limit to how much force it can take before it collapses. Let's say our table can take $F_{table}$ and the Earth exerts a force of $F_g$. Now if $F_{table} < F_g$ then the table can't exert a normal reaction greater than $F_{table}$ so there will be a net force down, breaking the table.

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  • $\begingroup$ Is it correct if I say that in a situation where the book is at rest and does not break the table, the book exerts mg on the table, and the table exerts N on the book, and if N = mg, the book is at rest, and vice-versa for the Earth (taking that the -mg on the Earth is balanced by -N)? And N is balancing mg in both cases but it's not a Newton's Third Law Pair? $\endgroup$ – rb2008 Aug 17 '18 at 3:17
  • $\begingroup$ The Earth exerts a force, $mg$, on the book and the table reacts with the normal force by keeping it fixed. So that $\sum F_i = 0 = mg -N $. Only if the table is strong enough to sustain that force. On your last question now I think of it, it is indeed not a case of the third law. To be able to use the third law the forces have to be of the same kind. So if I push against a wall, it pushes back. The Earth attracts the book but the book also attracts the Earth. $\endgroup$ – WarreG Aug 17 '18 at 7:16
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As you already mentioned correctly the Earth exerts the gravitational force $F_g=mg$ on the book. Now the ground has to exert the same force but in opposite direction on the book in order to prevent it from falling. This is the normal force. These are the only forces acting on the book. The book will exert the normal force in opposite direction on the ground by Newton’s third law. But it will not affect the books motion since it is not acting on it.

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