Question

Two identical books are stacked on a table. A third identical book is then placed on top of the first two, causing an increase in the normal force exerted by the bottom book on the middle book. Which of the following is a correct explanation for the increase in this normal force?

a) The gravitational force between the books increases.
b) The additional book exerts a downward force, increasing the normal force.
c) The table exerts an upward force, supporting the weight of the books.
d) The presence of the third book decreases the gravitational attraction between the first two.

Answer 1

The increase in the normal force exerted by the bottom book on the middle book when a third book is added is due to the additional downward force from the weight of the third book, requiring a greater upward force to balance it.

Step-by-step explanation:

When two identical books are stacked on a table, and a third identical book is placed on top of the first two, the normal force exerted by the bottom book on the middle book increases. The correct explanation for the increase in this normal force is (b) The additional book exerts a downward force, increasing the normal force. This is because the weight of the third book acts as additional downward force due to gravity, which increases the total force that the bottom book must counteract with an equal and opposite upward normal force to support the books in accordance with Newton's third law.

A normal force is a force exerted by a surface in response to the weight of an object resting on it, acting perpendicular to the surface. The normal force balances the gravitational force to prevent objects from falling through the surface. Therefore, when an additional book is added, the increased gravitational force must be balanced by an increased normal force from the bottom book to keep the stack stationary on the table.