# Question #849c2

##### 2 Answers

#### Answer:

The two you gave are a pair that obey Newton's third law

#### Explanation:

The normal force should be equal in magnitude but opposite in sign to the weight (weight is directed down, normal is directed up, and the forces must be equal in magnitude since F=N-W=ma, and a=0 so N=W), so the two forces you gave are a pair described by Newton's third law

#### Answer:

The normal contact force on the book and the weight of the book are not described by Newton's Third Law.

#### Explanation:

Newton's Third Law states that when object A exerts a force on object B, object B exerts an equal but opposite reaction force on object A.

A pair of forces here must satisfy:

- Equal in magnitude
- Opposite in direction
- Forces of the same type
- Should be acting on two different objects

Since both the weight and the normal contact force act on the book, these two forces are not considered a pair. Also, the two forces are not of the same type.

However, a pair in this situation would be the normal contact force exerted by the table on the book and the normal contact force exerted by the book on the table.

*Another pair of forces is the gravitational force on the book by the earth and the gravitational force on the earth by the book.