Newton's Third Law of Motion: Forces and Interactions

Chapter 5: Newton's Third Law of Motion

Introduction

This chapter focuses on Newton's Third Law of Motion, a fundamental principle in classical mechanics. Understanding this law is essential for analyzing forces and interactions between objects in various physical scenarios.

Forces and Interactions

Definition of Force Pairs

• Force is a push or pull exerted on an object, resulting from its interaction with another object.

• Every interaction requires a pair of forces acting on two different objects.

• Example: When a hand pushes against a wall, the hand exerts a force on the wall, and the wall exerts an equal and opposite force on the hand.

Newton's Third Law of Motion

Statement of the Law

• Newton's Third Law: Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first.

• This can be expressed mathematically as: $F_{AB} = -F_{BA}$ where $F_{AB}$ is the force exerted by object A on object B, and $F_{BA}$ is the force exerted by object B on object A.

Action and Reaction Forces

• One force is called the action force; the other is the reaction force.

• Action and reaction forces are co-pairs of a single interaction.

• Neither force exists without the other.

• They are equal in strength and opposite in direction.

• Action and reaction forces always act on different objects.

Examples and Applications

• Hand and Wall: You push on the wall (action); the wall pushes back on you (reaction).

• Hammer and Nail: The hammer exerts a force on the nail (action); the nail exerts an equal and opposite force on the hammer (reaction).

• Walking: Your foot pushes backward on the ground (action); the ground pushes your foot forward (reaction), allowing you to move.

Key Properties of Action-Reaction Pairs

• Action and reaction forces are always present together.

• They do not cancel each other because they act on different objects.

• Both forces are of the same type (e.g., contact, gravitational, electromagnetic).

Additional info:

• Newton's Third Law is crucial for understanding motion in systems ranging from simple collisions to rocket propulsion and even biological locomotion.