Question

A book is lying on the table. What is the angle between the action of the book on the table and weight of the book:
(A) 0\[{}^\circ \]
(B) 45\[{}^\circ \]
(C) 90\[{}^\circ \]
(D) 180\[{}^\circ \]

Answer 1

Hint:- Here the object is at rest on the table. We need to find the angle between the action of the book on the table and the weight of the book. We can use the concept of Newton’s third law of motion.

Complete step by step answer:
According to Newton’s third law, forces always occur in pairs. A single force cannot exist in the universe. When the book is lying on the table there are two forces acting on it. Since the book is on the table and it is not in direct contact with the earth, the gravitational force on it which is the weight acts downwards. Now, this weight is also being exerted on the table. So, following Newton’s third law an equal force called reaction acts on the book by the table and is acting upwards. So, the angle between the two forces is 180\[{}^\circ \].
Hence, the correct option is (D).

Additional Information:
Such forces are called action-reaction forces. Also, in case of uniform circular motion, a force acts towards the centre and is called centripetal force but there is no reaction force and to fulfil the criteria as per third law we say centrifugal force acts on the body and it has the tendency to move the body away from the centre.

Note:when some forces are acting on a body then the resultant force is given by adding them vectorially. Here one force is up while another force is down. Both are equal in magnitude but opposite in direction. So, they cancel out each other and body remains at rest