Why is a book on a table not an example of Newton's third law?

Question

My textbook explains Newton's Third Law like this: > If an object A exterts a force on object B, then object B exerts an equal but opposite force on object A It then says: > Newton's 3rd law applies in all situations and to all types of force. But the pair of forces are always the same type, eg both gravitational or both electrical. And: If you have a book on a table the book is exerted a force on the table (weight due to gravity), and the table reacts with an equal and opposite force. But the force acting on the table is due to gravity (is this the same as a gravitational force?), and the forcing acting from the table to the book is a reaction force. So one is a gravitational, and the other is not. Therefore this is not Newton's Third Law as the forces must be of the same type.

Answer 1

The most direct answer is that the reaction pair of forces in Newton's third law never act on the same system. Since both the gravitational and normal forces act on the book, they cannot be a reaction pair.

If we can describe a force as the force of "something" on "something else," it gets a bit easier to write the reaction pairs. For this case, we can call the gravitational force "the gravitational force of the Earth on the book." For the reaction force, just switch the order of the systems: "the gravitational force of the book on the Earth" (in other words, the book attracts the Earth with the same gravitational force as the Earth does the book).

The force of the table on the book, which is a normal force (contact force perpendicular to the surface of the table), is the "contact force of the table on the book." The reaction force to that would be the "contact force of the book on the table."

A key point here is that it takes more than just being equal in magnitude and opposite in direction for two forces to be a reaction pair. One must be CAUSED by the other, as well.