Question

1. 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 normal force on an object is equal in magnitude and opposite in direction to the force of gravity on that object.
b. the addition of the top book causes the force of gravity on the table to increase.
c. the forces of gravity on the bottom book and middle book increase because of the additional mass of the top book.
d. the third book produces an additional downward force on the middle book, thus increasing the upward force exerted by the bottom book to maintain equilibrium.

Explanation:

• Option a is incorrect because the normal force on the middle book is not just opposing its own gravity, but also the force from the top book.
• Option b is incorrect because the gravity on the table itself does not change; the total force on the table increases, but this does not explain the normal force between the bottom and middle book.
• Option c is incorrect because the gravitational force on the bottom and middle books depends only on their own mass, which does not change when adding the top book.
• Option d is correct: when the third book is added, it exerts a downward normal force on the middle book. For the middle book to stay in equilibrium, the upward normal force from the bottom book must increase to balance the total downward force (middle book's weight + top book's downward force).

Answer:

d. The third book produces an additional downward force on the middle book, thus increasing the upward force exerted by the bottom book to maintain equilibrium.