A book is at rest. What is the “reaction” force according to Newton’s third law to the gravitational force by the earth on the book?
A) The normal force exerted by the table on the book. B) The normal force exerted by the table on the ground C) The normal force exerted by the ground on the table. D) The gravitational force exerted on the earth by the book.
Hint: According to Newton’s third law of motion, forces always act in a pair, so if the earth exerts a gravitational force on the book, then the book will exert the same force but in the opposite direction. The two forces always act on different bodies.
Complete step by step solution: Newton’s third law states that if an object $1$ exerts a force on another object, say object $2$, then the object $2$ must exert a force of equal magnitude but in the opposite direction on object $1$. Forces always occur in pairs; a body doesn’t exert force on another body without experiencing a force on itself. When the book is kept on the table, its weight (gravitational force- the force with which the earth pulls the book) acts downwards, and a gravitational force with the same magnitude but opposite in direction is exerted on the book by the earth. The gravitational force exerted by the book on the earth is very small because the size of the earth is gigantic, so it does not affect the earth much.
The correct answer is (D), the gravitational force exerted on the earth by the book.
Additional information: The forces that are acting on the book are the normal force exerted by the table on the book in the upward direction, the gravitational force exerted by the earth that pulls the book towards itself.
Note: We will not consider the gravitational force exerted by the table on the earth and vice versa in this case, as we will only focus upon the forces acting on the book and also keep in mind that the reaction forces are equal and opposite in nature.