Question

a book rests on the shelf of a bookcase. the reaction force to the force of gravity acting on the book is

1. the weight of the book.
2. the frictional force between book and shelf.
3. the force of the shelf holding the book up.
4. none of these.
5. the force exerted by the book on the earth.

Explanation:

According to Newton's third - law, for every action, there is an equal and opposite reaction. The force of gravity acting on the book (the book is attracted to the Earth) has a reaction force which is the force exerted by the book on the Earth. The force of the shelf holding the book up is the normal force which balances the gravitational force on the book (a first - law equilibrium situation), not the reaction force to gravity. Friction is not relevant here as there is no relative motion or tendency of relative motion between the book and the shelf in the vertical direction. The weight of the book is just the magnitude of the gravitational force on the book, not the reaction force.

Answer:

1. The force exerted by the book on the earth.