# Confusion on Newton's 3rd law

## Main Question or Discussion Point

Confusion on Newton's 3rd law!!

According to the 3rd law, the equal and opposite forces are acting on two different bodies and therefore cannot cancel each other. They are mutual forces. Why then, so we say the force of gravity on say, a book on a table, and the normal force acting on the book by the table, is an example of the 3rd law? They are forces acting on one thing (the book) and are not triggered by each other. Also, hence they cancel each other out. So how is it an example of the third law?

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That is not an example of the 3rd law. The force pairs are:

1. weight of book towards Earth, and weight of Earth towards book
2. contact force on book from table, and contact force on table from book

The weight and contact forces of the book cancel for another reason: the elasticity of the table means that, as it is slightly deformed under the weight of the book, this deformation is accompanied by a restoring force upwards. The nature of this restoring force means that an equilibrium is reached between it and the book's weight.

Andrew Mason
Homework Helper

That is not an example of the 3rd law. The force pairs are:

1. weight of book towards Earth, and weight of Earth towards book
2. contact force on book from table, and contact force on table from book

The weight and contact forces of the book cancel for another reason: the elasticity of the table means that, as it is slightly deformed under the weight of the book, this deformation is accompanied by a restoring force upwards. The nature of this restoring force means that an equilibrium is reached between it and the book's weight.