In one line
For every action, there is an equal and opposite reaction.
Sir Isaac Newton, who wrote the three laws of motion in 1687.
Image: Wikipedia
The rule
Newton's Third Law of Motion states that for every action, there is an equal and opposite reaction. This means that forces always occur in pairs; when one object exerts a force on another, the second object exerts a force of equal magnitude and opposite direction on the first object.
The formula is: F₁ = −F₂
- F₁: Force by object 1 on object 2
- F₂: Force by object 2 on object 1
- The minus sign (−) means the forces are in opposite directions.
Let's understand it simply
Imagine you are standing on a skateboard. If you push a wall, you roll backwards. You did not move the wall, but you moved! Why? Because when you push the wall (action), the wall pushes you back (reaction) with the same force, but in the opposite direction.
Think of jumping off a small boat onto the shore. As you jump forward (action), the boat moves backward (reaction). Both you and the boat feel a force, but in opposite directions.
How it works
- You apply a force to something (like pushing a ball).
- That thing applies the same force back to you, but in the opposite direction.
- Both forces happen at the same time, not one after the other.
- The two forces are always equal in size (magnitude).
- The two forces act on different objects (not on the same object).
- These pairs of forces are called "action" and "reaction" forces.
- You push something (action)
- It pushes you back (reaction)
- Both forces are equal and opposite
- Forces act on different objects
The story behind it
Sir Isaac Newton, a scientist from England, wrote this law in 1687 in his book "Principia Mathematica." He wanted to explain why things move the way they do, especially when forces are involved. Newton noticed that whenever two objects interact, they always push or pull on each other equally, but in opposite ways. This law helped people understand everything from how rockets launch to why we can walk.
Where you see it in real life
- Walking: When you push your foot backward on the ground (action), the ground pushes you forward (reaction), helping you move.
- Rowing a boat: The oar pushes water backward (action), and water pushes the boat forward (reaction).
- Cricket bat and ball: The bat hits the ball (action), and the ball pushes back on the bat (reaction). That's why your hands feel the impact.
- Jumping off a bus step: You push down on the step (action), the step pushes you up (reaction), so you can jump off.
- Rocket launch: The rocket pushes hot gases down (action), and the gases push the rocket up (reaction).
- Inflating a balloon and letting it go: Air rushes out one way (action), and the balloon flies the other way (reaction).
Why it matters
- Explains movement: It helps us understand how we walk, run, swim, or drive.
- Designing vehicles: Engineers use it to design cars, rockets, and boats.
- Sports: It explains how bats, rackets, and balls work in cricket, tennis, or football.
- Safety: Knowing about action and reaction helps in making safer helmets, shoes, and vehicles.
- Exams: Many physics questions, especially in CBSE and other boards, ask about this law and its examples.
Worked example
A 50 kg student jumps off a boat. The student jumps forward with a force of 200 N. What force does the boat feel, and in which direction?
Given: Force by student on boat = 200 N (forward)
Step 1: Write the rule F₁ = −F₂
Step 2: Action: Student pushes boat forward (200 N)
Step 3: Reaction: Boat pushes student backward (−200 N)
Step 4: So, boat feels 200 N force in the opposite direction (backward)Common mistakes
- Thinking action and reaction act on the same object: They always act on different objects.
- Believing one force comes after the other: Both happen at the same time.
- Forgetting direction: The two forces are always in opposite directions.
- Missing the pair: Every force has a partner — never just one force alone.
Remember it
"Push and get pushed back — always equal, always opposite."
Check yourself
If you push a wall with 50 N, how much does the wall push you back?
50 N, in the opposite direction, because of Newton's third law.
Why does a rocket move up when its engines push gases down?
The gases push the rocket up with equal force in the opposite direction (reaction).
Do action and reaction forces act on the same object? Why or why not?
No, they act on different objects. That's why they do not cancel each other.
When you jump off a boat, why does the boat move backward?
Because your jump (action) pushes the boat back (reaction) with equal force.
Quick recap
- Newton's third law: Every action has an equal and opposite reaction.
- Forces always come in pairs, acting on different objects.
- Both forces happen at the same time and are equal in size.
- You see this law in walking, rowing, cricket, and rockets.
- Never forget: action and reaction are always opposite in direction.