Question

A semicircular stack of books with the first five books leaning on one another and the other books standing straight up.
Why will all of the books in this line of books fall over? Check all that apply.

Each book is too close to the next book.
Each book pushes the next book.
Each book transfers energy to the next book.
Each book pulls the next book.
Each book does work on the next book.

Answer 1

Choose:
- Each book pushes the next book. — Yes. A falling book exerts a contact force (push) on the next book, producing a torque that tips it.
- Each book transfers energy to the next book. — Yes. Gravitational potential energy (and kinetic energy of the falling book) is passed to the next book, allowing it to rotate.
- Each book does work on the next book. — Yes. The contact force acts through a displacement at the contact point, so the falling book does work on the next one.

Not chosen:
- Each book is too close to the next book. — No (being close can make contact more likely, but “too close” by itself is not the physical cause).
- Each book pulls the next book. — No. The interaction is a push (contact force), not a pull.