What is the Third Law of Motion?

S4-SA1-0247

Grade Level:

Class 6

Space Technology, EVs, Climate Change, Biotechnology, HealthTech, Robotics, Chemistry, Physics

Definition

What is it?

Newton's Third Law of Motion tells us that for every action, there is an equal and opposite reaction. This means that whenever one object pushes or pulls another object, the second object pushes or pulls back with the same strength but in the opposite direction.

Simple Example

Quick Example

Imagine you are playing gully cricket and hit a ball with your bat. When your bat hits the ball (action), the ball also pushes back on your bat (reaction). This push-back is why you might feel a slight jolt in your hands, and it's what makes the ball fly in the opposite direction.

Worked Example

Step-by-Step

Let's say a rocket is launching into space. --- Step 1: The rocket engines push hot gases downwards with a force of 10,000 Newtons. This is the 'action'. --- Step 2: According to Newton's Third Law, the ground (or the air) pushes back on the rocket with an equal force of 10,000 Newtons. This is the 'reaction'. --- Step 3: This upward reaction force is what lifts the heavy rocket off the ground and sends it into the sky. --- Answer: The rocket moves upwards because of the equal and opposite reaction force from the expelled gases.

Why It Matters

Understanding this law is super important for designing rockets for ISRO's space missions and building safe cars for everyday travel. Engineers use this principle in robotics to make robots move efficiently and in healthtech to design prosthetics that work naturally.

Common Mistakes

MISTAKE: Thinking that action and reaction forces cancel each other out because they are equal and opposite. | CORRECTION: Action and reaction forces act on DIFFERENT objects, so they don't cancel each other. The bat pushes the ball, and the ball pushes the bat.

MISTAKE: Believing that the action force always happens before the reaction force. | CORRECTION: Action and reaction forces happen AT THE SAME TIME. They are simultaneous, like pressing a button and the light turning on.

MISTAKE: Confusing action-reaction pairs with balanced forces. | CORRECTION: Balanced forces act on the SAME object and cancel out. Action-reaction pairs act on DIFFERENT objects and cause motion or change.

Practice Questions

Try It Yourself

QUESTION: When you push a wall, what is the reaction force? | ANSWER: The wall pushes back on you with an equal and opposite force.

QUESTION: A swimmer pushes water backward with a force of 50 Newtons. What is the force exerted by the water on the swimmer, and in which direction? | ANSWER: The water pushes the swimmer forward with a force of 50 Newtons.

QUESTION: A book rests on a table. Identify the action-reaction pair involving the book and the table. If the book weighs 10 Newtons, what is the force the table exerts on the book? | ANSWER: Action: Book pushes down on the table with 10 Newtons. Reaction: Table pushes up on the book with 10 Newtons.

MCQ

Quick Quiz

Which of these is an example of Newton's Third Law of Motion?

A cricket ball slowing down due to friction

A car accelerating when the engine pushes it forward

A boat moving forward as oars push water backward

A heavy box being harder to move than a light box

The Correct Answer Is:

Option C shows an action (oars pushing water backward) and an equal and opposite reaction (water pushing the boat forward). The other options relate to other laws of motion or concepts like friction and inertia.

Real World Connection

In the Real World

Think about how a jet plane takes off from an airport like Delhi's IGI. Its engines forcefully push hot air backward (action). The air, in turn, pushes the plane forward with an equal and opposite force (reaction), allowing the massive aircraft to gain speed and lift into the sky. This same principle is used by ISRO to launch satellites.

Key Vocabulary

Key Terms

ACTION: A force exerted by one object on another. | REACTION: An equal and opposite force exerted by the second object back on the first. | FORCE: A push or a pull that can change an object's motion. | NEWTON: The unit used to measure force.

What's Next

What to Learn Next

Great job learning about Newton's Third Law! Next, you can explore 'What is Momentum?' This concept builds on forces and motion, helping you understand why some moving objects are harder to stop than others. Keep up the amazing work!