🥋 Newton’s Third Law – White to Black Belt Mastery

“For every action, there is an equal and opposite reaction.”

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White Belt Level – Key Concept

Newton’s Third Law says that forces always come in pairs.
If object A pushes on object B, then object B pushes back on object A with equal strength but opposite direction.

In simple words: Action = Reaction.

🎮 Explore: PhET Simulations – Forces and Motion: Basics

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Core Principles

1. Forces always come in pairs (action and reaction).
2. The two forces are equal in size but opposite in direction.
3. They act on different objects (not on the same object).
4. Motion depends on the mass of each object (Newton’s Second Law still applies).

📖 Learn more: OpenStax University Physics, Ch. 4

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Key Equations (SI Units)

• Action Force: F⃗_AB (force of A on B)
• Reaction Force: F⃗_BA (force of B on A)
• Rule: F⃗_AB = –F⃗_BA

🔎 Reference: HyperPhysics – Newton’s Third Law

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Common Mistakes and Pitfalls

• Thinking forces cancel each other out (they act on different objects, so they don’t).
• Forgetting to apply Newton’s Second Law to each object separately.
• Mixing up action and reaction pairs (they must always be the same type of force: contact-contact, gravity-gravity, etc.).

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Sensei’s Shortcuts

🤜 Push-Back Test: If you feel a push when you push on something, that’s the reaction force. Try it with a wall or desk.

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Worked Example – Step by Step (White Belt)

Problem: A swimmer pushes the water backward with a force of 120 N. What force does the water exert on the swimmer?

Step 1. Formula:
F⃗_AB = –F⃗_BA

Step 2. Substitution:
Swimmer on water = 120 N backward
Water on swimmer = 120 N forward

Step 3. Final Answer:
The water pushes forward on the swimmer with 120 N.

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Practice Drill (White Belt)

1. A horse pulls on a cart with 500 N. How much force does the cart pull on the horse?
   Answer: 500 N (opposite direction)
2. You push a wall with 50 N. What force does the wall push back with?
   Answer: 50 N (opposite direction)

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Yellow Belt Extension – Deeper Skills

Problem: A 60.0 kg person jumps off a boat by pushing backward on it with 300 N. What happens to the boat (mass 240 kg)?

Solution Outline:

• Action: person pushes boat = 300 N backward
• Reaction: boat pushes person = 300 N forward
• Person’s acceleration: a = F/m = 300 / 60.0 = 5.00 m/s² forward
• Boat’s acceleration: a = F/m = 300 / 240 = 1.25 m/s² backward

Final Answer:
Person: 5.00 m/s² forward
Boat: 1.25 m/s² backward

🎮 Try this in the Motion tab of PhET Forces and Motion: Basics.

📖 Extra reading: Khan Academy – Newton’s Third Law

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Black Belt Mastery – Exam Strategy and Challenge

Challenge Problem: A 70.0 kg astronaut in space pushes a 500 kg satellite with a force of 200 N for 2.00 s.

a) What is the astronaut’s acceleration and final velocity?
b) What is the satellite’s acceleration and final velocity?
c) Who moves faster and why?

Strategy Notes:

• Use Newton’s Third Law: forces equal and opposite.
• Apply F = m a to each object separately.
• Use v = a t for velocity change.

Solution:
a) Astronaut:
a = F / m = 200 / 70.0 = 2.86 m/s²
v = a × t = 2.86 × 2.00 = 5.72 m/s (backward)

b) Satellite:
a = F / m = 200 / 500 = 0.400 m/s²
v = a × t = 0.400 × 2.00 = 0.800 m/s (forward)

c) Astronaut moves faster because of smaller mass.

Final Answers:
a) Astronaut v = 5.72 m/s backward
b) Satellite v = 0.800 m/s forward
c) Astronaut moves faster

🚀 Related: NASA – Newton’s Laws in Space

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Sensei’s Final Words

Newton’s Third Law reminds you that forces always come in pairs. Every push has a push-back. The secret is knowing which object each force acts on. Train your mind to always ask: Who is pushing who?