Unit 2.3: Newton’s Third Law

Introduction

Newton’s Third Law describes the nature of forces as interactions between two objects. This law states that for every action, there is an equal and opposite reaction. This principle is fundamental in understanding motion, force interactions, and conservation laws.

Key Concepts

Newton’s Third Law: If object A exerts a force on object B, then object B exerts an equal and opposite force on object A.
Action-Reaction Pairs: Forces always come in pairs that are equal in magnitude but opposite in direction.
Forces Act on Different Objects: The two forces in an action-reaction pair never act on the same object.

Tip: Always identify the two objects interacting when applying Newton’s Third Law. The forces are equal but act on different objects.

Examples of Newton’s Third Law

Walking: You push backward on the ground, and the ground pushes you forward.
Rocket Propulsion: The rocket expels gas downward, and the gas pushes the rocket upward.
Jumping: You push down on the ground, and the ground pushes you upward.
Recoil: When firing a gun, the bullet moves forward while the gun recoils backward.

Mathematical Applications

Newton’s Third Law does not require mathematical calculations directly but is crucial for analyzing force interactions in problems. In force diagrams, every force must have a corresponding reaction force acting on a different object.

Common Misconception: The action-reaction forces do not cancel out because they act on different objects.

Practice Activities

Activity 1: Identifying Action-Reaction Pairs

Describe the action-reaction forces in the following scenarios:

A book resting on a table
A person stepping off a boat onto a dock
A fish swimming in water

Activity 2: Free-Body Diagram Challenge

Draw a free-body diagram for a box sitting on the floor. Identify all forces acting on the box and their reaction forces.

Summary & Exam Preparation Tips

Newton’s Third Law is essential for understanding forces and motion. Key takeaways:

Forces always occur in pairs—one force is the action, the other is the reaction.
Action-reaction forces are equal in magnitude but opposite in direction.
Action-reaction forces act on different objects and do not cancel out.
Identifying force pairs is crucial for solving force-related physics problems.

Understanding Newton’s Third Law will help with analyzing motion and interactions between objects in real-world applications and AP Physics problems.