Question

A physics book rests on a table top. A gravitational force pulls down on the book According to Newton's third law. The force that is equal to this gravitational force is:
(A) The force the table exerts upward on the book.
(B) The force the book exerts downward on the table.
(C) The force the table exerts downward on the table.
(D) The force the book exerts upward on the table.
(E) The force the air pressure exerts down on the book.

Answer 1

Hint:Here the book is at rest. We know from Newton’s first law that if a body is at rest it will remain at rest, if a body is in motion it will remain in motion until and unless some external force acts on it. So, one thing is clear that all the forces which are acting on the book, their resultant comes out to be zero and because of that the book is at rest. The force of gravity always acts downwards.

Complete answer:
 We know that according to Newton's third law of motion every action has an equal and opposite reaction. Here physics book is in contact with the table. The force exerted on the table is due to the gravitational pull of the book towards the earth that is the weight of the book.
Therefore the table exerts an opposite force (upward force) on the book which is equal to the gravitational force. The gravitational force is balanced by the force exerted by the table on the physics book and so the book remains at rest and does not fall down.The force that is equal to the gravitational force is the force the table exerts upward on the book.

So, the correct option is A.

Note:One should be familiar with the three laws of Newton. Here the third law is implemented. The books on the table exert equal and opposite forces on each other. 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. So, following Newton’s third law an equal force called reaction acts on the book by the table and is acting upwards.