# Third Law of Motion:Newton

1. Jun 4, 2012

### moatasim23

Action and reaction do not balance each other.But then why a book lying on a table lies at rest?The reaction force and the gravitational force cannot balance each other.Then how is it?

2. Jun 4, 2012

### Staff: Mentor

Action and reaction forces act on different bodies, so they don't 'balance'. The book is in equilibrium because the forces acting on the book add up to zero (they 'balance'). But those forces are not action/reaction pairs.

The first thing you need to do is identify all the forces acting on the book and their third law pairs.

3. Jun 5, 2012

### moatasim23

Can u help me to identify them?I can only think of two forces.
THANKS.

4. Jun 5, 2012

### Staff: Mentor

OK, what are those two forces? And what bodies exert those forces? (That will help you identify the 3rd law pairs.)

5. Jun 5, 2012

### tiny-tim

hi moatasim23!

sorry, but i don't understand any of your post
action and reaction are always equal and opposite
they do balance each other

what makes you think that they don't?

6. Jun 5, 2012

### moatasim23

Gravitational force and the upward normal reaction force of the table.

7. Jun 5, 2012

### Staff: Mentor

Good. Those two forces happen to be equal and opposite, thus the book is in equilibrium. But those two forces are not 3rd law pairs (action/reaction pairs).

The gravitational force on the book is exerted by the earth. The 3rd law pair to that force is an equal and opposite gravitational force exerted on the earth by the book. (They attract each other.)

The upward force of the table on the book is exerted by the table. The 3rd law pair to that force is an equal and opposite force exerted on the table by the book. (They push against each other.)

8. Jun 6, 2012

### moatasim23

Thanks DOC AL.This question was confusing me for a lot time.Thanks a lot.