When an object is at rest, the magnitudes of the pair forces acting on it are equal and their directions are opposite. This is a fundamental principle of physics known as Newton's Third Law of Motion.

Here's a breakdown:

* Newton's Third Law: For every action, there is an equal and opposite reaction.

* Pair Forces: These are forces that always occur in pairs. When one object exerts a force on another object, the second object exerts an equal and opposite force back on the first.

* Rest: An object at rest means it has zero acceleration, implying that the net force acting on it is zero.

Example:

Imagine a book sitting on a table. The book exerts a downward force due to gravity on the table. Simultaneously, the table exerts an upward force on the book (the normal force). These forces are equal in magnitude and opposite in direction.

Why this matters:

If the forces were not equal and opposite, the book would either move downwards (if gravity was stronger) or upwards (if the normal force was stronger). Since the book is at rest, the forces must be balanced.

Important Note: It's crucial to understand that these forces act on *different* objects. One force acts on the table, while the other acts on the book.