Exploring Newton's Third Law of Motion

Today's focus is on Newton's Third Law of Motion, an important topic for students studying the laws of motion in Class 11 and AP Physics. We'll explore common questions: What is Newton's Third Law? How does it manifest in action-reaction pairs? And how do we identify these pairs in various scenarios?

Understanding Newton's Third Law

Newton's Third Law reveals a fundamental symmetry in nature: forces always occur in pairs. This law posits that when a body exerts a force on another, it experiences an equal and opposite force in return. A key inquiry here is: Why does force always occur in pairs?

Example 1: A Book and a Wall

Consider a book against a wall. Newton's Third Law explains this through the forces "Force of book on the wall" and "Force of wall on the book," where F_wall on book = - F_book on wall.

Example 2: A Box on a Table

In a scenario with a box on a table, the forces involved are "Force of box on the table" and "Force of table on the box," described by F_table on box = - F_box on table. This example highlights the equal and opposite nature of these forces.

Note on Action-Reaction Pairs

A crucial point in Newton's Third Law is that these force pairs never act on the same body, explaining why force pairs act on different bodies.

Example 3: Earth and a Ball

A non-contact example involves Earth and a ball. The forces here are "Force due to Earth on the ball" and "Force due to ball on Earth," following the equation F_Earth on ball = - F_ball on Earth.

Summary of Newton's Third Law

Newton's Third Law of Motion states that for every action, there's an equal and opposite reaction. This is exemplified by the interaction between a book and a wall, where the force exerted by the book is met with an equal and opposite force by the wall. It highlights the symmetry of forces in nature, always appearing in action-reaction pairs.

Newton's Third Law: Key Moments

00:00 - Introduction to Newton's Third Law
00:07 - Exploring Why Force Always Occurs in Pairs
00:18 - Example 1: Book Leaning Against a Wall
01:09 - Understanding Equal and Opposite Forces
01:50 - Example 2: Box on a Table
02:35 - Defining Action-Reaction Pairs in Newton's Third Law
03:24 - Why Force Pairs Act on Different Bodies
03:57 - Example 3: Gravity Between Earth and a Ball
05:15 - Equal and Opposite Forces Without Physical Contact
05:51 - Problem-Solving Approach for Two Boxes on a Table
07:06 - Importance of Free Body Diagrams
08:26 - Determining Forces and Acceleration
09:54 - Calculating Acceleration and Interaction Force
10:15 - Methods to Verify Results
11:13 - Clarifying Misconceptions About Force Transmission

Calculations for Earth's Movement

Let's consider the movement of Earth due to an apple falling from a height of 100 meters, with the apple's mass being 0.5 kg, and Earth's mass approximately 6 × 10²⁴ kg.

1. Time of Impact: Using the motion equation ½gt² = h, we find t ≈ 4.5 seconds.
2. Forces in Action: Gravitational force by Earth on the apple is 5 Newtons, and by Newton's third law, Earth experiences an equal and opposite force.
3. Earth's Reaction: Earth's acceleration towards the apple is a minuscule 8 × 10⁻²⁵ m/s².
4. Earth's Displacement: The Earth moves a negligible distance of about 8 × 10⁻²⁴ meters towards the apple in 4.5 seconds.

This exploration into Newton's Third Law provides a deep understanding of the law's implications and its role in the mechanics of interactions in the physical world, an essential topic for Class 11 and AP Physics students.