Question

Newton’s third law of motion is often summarised as ‘Every action has an equal and opposite reaction’.
A book rests on a table. If the weight of the book is the ‘action’ force what is the ‘reaction’ force?
A. the pull of the book on the Earth
B. the pull of the Earth on the book
C. the push of the book on the table
D. push of the table on the book

Answer 1

Hint: Newton’s third law of motion states that all the forces between two objects exist in equal magnitude and opposite direction. Newton also used this law to derive the law of conservation of momentum.
Formula Used: ${\vec F_{AB}} = - {\vec F_{BA}}$

Complete answer:
Force is defined as the push or a pull acting on an object resulting in its interaction with another object. Newton’s third law of motion states that when one body exerts a force on the other body, the first body experiences a force which is equal in magnitude and opposite in the direction of the force which is exerted on it.
In other words, it states that when an object A exerts a force on object B, then the object B must exert a force of equal magnitude and opposite direction on the object A.
Mathematically it can be given as, ${\vec F_{AB}} = - {\vec F_{BA}}$
According to the question, the book applies a force on the table which is equal to its weight. Thus, by applying Newton's third law of motion the table will also apply the same force on the book.
Therefore, the force is the push of the table on the book.

Hence, option (D) is the correct answer.

Note:
For a better understanding of the concept of Newton’s third law we can see some applications of it. Some examples of interaction of force pairs are:
1) The flight of birds is considered as an example of an action – reaction pair. The wings of the birds push the air downwards and the air pushes the birds upwards.
2) While swimming, a swimmer pushes against the water and the water pushes back on the swimmer.