# PHYS 2211 Module 5.5

## Newton’s Third Law 5.5 Newton’s Third Law

### Learning Objectives

By the end of this section, you will be able to:

• State Newton’s third law of motion
• Identify the action and reaction forces in different situations
• Apply Newton’s third law to define systems and solve problems of motion

### Newton’s Third Law

Whenever one body exerts a force on a second body, the first body experiences a force that is equal in magnitude and opposite in direction to the force that it exerts.

“Every action has an equal and opposite reaction.” Discuss!

Consider how you would answer these questions. Then bring this to class for a group discussion.

Identify the action and reaction forces in the following situations:

• Earth attracts the Moon
• a person kicks a football
• a rocket accelerates upward
• a car accelerates forward
• a high jumper leaps
• a bullet is shot from a gun Practice! Discuss!

Consider how you would answer these questions. Then bring this to class for a group discussion.

A passenger is riding in an elevator. The passenger weighs 650 N and the floor of the elevator exerts a force of 620 N on the passenger.

(a) What are the reaction forces to these two forces?

(b) Is the passenger accelerating?

(c) If so, what are the magnitude and direction of the acceleration?