Question

A book is lying on the table. What is the angle between the action of the book on the table and the weight of the table?
(A) $0^\circ$
(B) $45^\circ$
(C) $90^\circ$
(D) $180^\circ$

Answer 1

The book is static, hence it in equilibrium. Then the only force acting on the table due to the book is the weight of the book.

Complete answer:
A book is said to lie on a table. The angle between the action of the book on the table and the weight of the book is to be found. The statement “action of the book on the table” signifies an action force the book exerts on the table.
Generally, action forces can be said to be forces which may act on their own, as a result of a field, or biological agents such as humans and animals. Examples of action forces are weight of a body, electrostatic attraction and repulsion, and others.
On the other hand, reaction forces can be said to be forces which do not exist without the presence of an action force. They exist to oppose after an action force acts upon something. Reaction forces can be the normal force, friction, tension, etc.
Now, since the book lies (and not moving), then the only force on the table due to the book is the weight of the book. Hence, this can be considered the action of the book. Since, the weight of the book and the action of the book on the table act in the same direction, then the angle is zero.
Thus, the correct option is A.

Note:
Note that the definitions of action and reaction forces given are based on macroscopic point of view. All forces are a result of one force field or another. Tension for example, is a result of the force of attraction between the molecules of the string, hence when displaced (stretched), tend to return to their initial position. In reality, any kind of force can be a reactive force.