Force Pairs Anchor Chart
Physics Anchor Chart Part 1 • Core Principles
Newton's 3rd Law
Newton's Third Law of Motion
The Law of Action & Reaction Force Pairs
The Golden Rule \( \vec{F}_{A \text{ on } B} = -\vec{F}_{B \text{ on } A} \)
“For every ACTION force, there is an EQUAL and OPPOSITE REACTION force.”
4 Must-Know Facts About Force Pairs
1
Equal in Size (Magnitude)
Both forces have the exact same strength. If you push a wall with 50 N, the wall pushes you back with 50 N!
2
Opposite in Direction
The forces push or pull in exactly reverse directions. One pushes FORWARDS (→), the other pushes BACKWARDS (←).
3
Two Different Objects! KEY
Forces NEVER act on the same object. Object A pushes Object B, and Object B pushes Object A!
4
Simultaneous (Same Time)
They happen at the exact same instant. Neither force starts first—they exist together like two sides of a coin.
See It in Action: Real-World Examples
1. Skateboarder 🛹
Foot pushes Ground ←
Ground pushes Foot →
Result: The ground pushes the skater forward!
2. Rocket Launch 🚀
Engines push Gas ↓
Gas pushes Rocket ↑
Result: Hot gas down, rocket accelerates up!
3. Swimmer 🏊
Hands push Water ←
Water pushes Hands →
Result: Water pushes swimmer forward!
Sentence Frame Scaffold (Use this every time!) Problem-Solving Tool
[Object A] pushes [Direction] on [Object B]
↕ EQUAL & OPPOSITE ↕
[Object B] pushes [Opposite Direction] on [Object A]
Common Trap: Why Don't the Forces Cancel Out?
Forces can only cancel out if they push on the same single object (like a tug-of-war on one rope). Newton's Third Law forces act on TWO DIFFERENT objects, so they never cancel!
Same Force Size | Opposite Direction | Two Different Objects
Physics Anchor Chart Part 2 • Visual Models & Scenarios
Deep Dive Models
Visual Models: Newton's Third Law in Action
Comparing Masses, Directions, and Force Pair Interactions
A
Model 1: The Unequal Mass Paradox (Same Force ≠ Same Motion!)
Newton's 2nd & 3rd Laws Combined
💣 Heavy Cannon (500 kg)
← Recoil Force: 2,000 N
Big Mass = Small Acceleration
(Small backward kick)
⚪ Light Ball (5 kg)
Forward Force: 2,000 N →
Small Mass = HUGE Acceleration
(Shoots out super fast!)
SPED Rule: The forces are 100% EQUAL (2000 N = 2000 N), but the lighter object flies faster!
B
Model 2: Spot the Difference! (Balanced Forces vs. Force Pairs)
NOT a 3rd Law Pair 1 Object
Book Sitting at Rest on Table
↑ Table pushes UP on book (Normal)
↓ Earth pulls DOWN on book (Gravity)
Why not? Both forces push/pull on the SAME single book. They cancel out (Net Force = 0).
TRUE 3rd Law Pair 2 Objects
Book & Table Contact Interaction
↓ Book pushes DOWN on Table
↑ Table pushes UP on Book
Why yes! Force acts on TWO different objects (Table & Book). They never cancel!
C
Model 3: Walking & Jumping (How We Move)
👟
Walking on Pavement
• Foot pushes Ground ← (backward)
• Ground pushes Foot → (forward)
*Why you slip on ice: No grip means you can't push back, so ice can't push you forward!
🤸
Jumping Off the Floor
• Feet push Floor ↓ (downward)
• Floor pushes Feet ↑ (upward)
*The harder your legs push down, the higher the floor propels you up!
The 3-Step "FLIP-IT" Strategy for Any Test Question SPED Step-by-Step
Step 1: Circle 2 Things
Find the two interacting objects (e.g., Baseball Bat & Ball).
Step 2: State Action
State direction: "Bat hits Ball FORWARD with 800 N."
Step 3: Flip & Reverse
Swap names & opposite direction: "Ball hits Bat BACKWARD with 800 N."
Remember: Newton's 3rd Law is always an exchange between TWO objects! Equal Magnitude • Opposite Direction
