Assuming I have a book on a table. First, there is gravitational force acting on the book, which causes the book to also exert the same amount of force on the Earth. Now, the table will also exert a normal force on the book to counteract the gravitational force (weight of the book). However, there must be a reaction force to the normal force isn't it? If so, how can the book be possibly staying on the table if it exerts another reaction force on the table?
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This sentence "However, there must be a reaction force to the normal force isn't it?" is about a force on the table and not the book.
The book has the gravitational pull of the earth acting on it and the normal reaction force from the table. Those two balance and the book stays on the table.
answered Jul 2, 2021 at 14:52
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$\begingroup$ All forces act in pairs according to Newton's 3rd law. Therefore, the normal force must also have a reaction force. What is the reaction force of the normal force? $\endgroup$ Jul 5, 2021 at 16:37
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2$\begingroup$ Best to swap words to check you've got a Newton's 3rd law pair "the table pushes the book" swaps to "the book pushes the table" - is one pair. "the earth pulls the book" swaps to "the book pulls the earth" is another pair. It ends up with two forces acting on the book. In this sentence " the table will also exert a normal force on the book to counteract the gravitational force (weight of the book)" it's true that the forces balance, the table might bend until they do, but they aren't a Newtons 3rd law pair. In a N3 pair the forces act on different objects $\endgroup$ Jul 5, 2021 at 19:23