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The gravitational force is a fundamental interaction between the book and Earth. It pulls the book downward toward the Earth's center. This force is due to the mass of the book interacting with Earth's gravitational field. In physics, we express the gravitational force using the formula: \[ F_g = m imes g \] where \( m \) is the mass of the book and \( g \) is the acceleration due to gravity, approximately \( 9.8 \text{ m/s}^2 \) on Earth's surface.
This force is always directed downward, no matter where the object is located.
Newton's Third Law tells us that for every force, there is an equal and opposite reaction force. For the book, the reaction to its weight pulling it down is the book exerting a gravitational pull upward on Earth. However, due to Earth's massive size, this pull is imperceptible.

The normal force is the force exerted by a surface to support the weight of an object resting on it. It's called 'normal' because it acts perpendicular to the surface. For our book, the normal force acts upward, counteracting the gravitational force pulling it down.
Importantly, the normal force is not always equal to the weight of the object. It adjusts based on the presence of other forces or movement, if any exist. However, in this situation, the book is at rest on a horizontal surface, making the normal force equal in magnitude to the gravitational force.
The formula we use to describe the normal force in this context is simply: \[ F_n = F_g \] where \( F_n \) is the normal force, and \( F_g \) is the gravitational force. As for the reaction, when the surface exerts the normal force upward on the book, the book pushes back equally downward onto the surface, adhering to Newton's Third Law.

The concept of a reaction force stems from Newton's Third Law, which states that every action has an equal and opposite reaction. For the book on the table, each force interacting with the book has a corresponding reaction force.

• Gravitational Force Reaction: The Earth's gravitational pull on the book is matched by the book exerting an equal upward gravitational force on Earth.
  However, due to the massive difference in mass, this upward force doesn't affect Earth visibly.
• Normal Force Reaction: The upward normal force that the surface applies to the book is counteracted by the book pressing downward on the surface with equal force.
  This balanced interaction ensures the book stays at rest without penetrating the surface.

These reaction forces are not direct contact forces but demonstrate the conservation and balance inherent in physical interactions.