Part 2 · The Critical Distinction

Same equation.
Different physics.

Equilibrium:  F₁ = −F₂  — two forces on one body, result: rest.
Interaction:  FAB = −FBA  — forces on two different bodies, always paired.

The algebra is identical. The physics context is completely different.
Your mission: master this distinction so no question can fool you.

E2 · The Core Distinction

Equilibrium vs Interaction

Both use the same vector equation. The critical difference: which bodies do the forces act on?

Criterion ⚖ Equilibrium Condition 🔄 Principle of Interaction
Equation F₁ = −F₂ FAB = −FBA
BodiesONE body — both forces on the same objectTWO bodies — each force on a different object
EffectZero net force → object at rest or constant velocityAlways exist in pairs — one cannot exist without the other
Can cancel?✓ Yes — they cancel → equilibrium✗ No — different bodies → cannot cancel each other
Classic trapW and N on a book are an EQUILIBRIUM pair (both on book) — NOT an interaction pair. The interaction partner of N is the force the book exerts on the table, which acts on the TABLE.
✓ The Two-Question Test
Q1: Are both forces on the same body?
→ YES → could be an equilibrium pair

Q2: Do the forces act on different bodies and always coexist?
→ YES → it is an interaction pair
⚠ Most Common Misconception
Student says: "N and W on a book are an interaction pair because N = −W."

WRONG. Both N and W act on the book → equilibrium pair.

The true interaction pair: F(table→book) ↑ on book  ↔  F(book→table) ↓ on table.
E3 · Activity 1 — Force Painter

Draw the Forces

A book rests on a table. Select a force tool, then tap the correct origin point on the diagram to place each arrow. The system checks your placement.

Equilibrium forces (on book):
W ↓ from centre G · N ↑ from bottom contact
Interaction force (on table):
F(book→table) ↓ from top surface of table
Tool:
Select a tool then tap the diagram at the correct origin point
E3 · Activity 2 — Sorting Arena

Sorting Arena

Eight force pairs are shuffled below. Drag each card into the correct zone. On mobile: tap a card to select it (it highlights), then tap a zone to place it. Tap a placed card to return it.

Force pair cards — drag or tap to sort ↓
⚖ Equilibrium Pair
F₁ = −F₂  ·  Same body
🔄 Interaction Pair
FAB = −FBA  ·  Two bodies
E3 · Activity 3 — Body Separator

Body Separator

A ball rests on a table. Drag the slider to physically separate the two bodies. Watch exactly which forces stay with each body — and identify the pairs.

Together Separated
Drag the slider right to begin separating the two bodies.
Quick Check · After Separating
When the ball is fully isolated, which two forces act ON THE BALL?
AW ↓ and F(table→ball) ↑ — both on ball → equilibrium pair
BW ↓ and F(ball→table) ↓ — both downward, reaction pair
COnly W ↓ — nothing else acts on the ball
E4 · Compound Challenge

The Verdict Chamber

A three-stage compound challenge. Complete each stage to unlock the next. All stages are graded.

1
Identify Forces →
2
Classify Pairs →
3
Write Verdict
0 / 15 pts
Scenario

A magnet hangs from a ceiling by a string. A steel ball rests on the floor below — the magnet pulls the ball magnetically but not enough to lift it. The ball stays on the floor.

Tap each force label to identify which body it acts on:

W(magnet) ↓
T(string→magnet) ↑
Fmag(magnet→ball) ↑
W(ball) ↓
N(floor→ball) ↑
0 / 5 identified

Stage 2 — Classify the Pairs

Tap a pair to select it, then tap a classification button:

Stage 3 — Write the Verdict

In one precise sentence, explain why N(floor→ball) ↑ and W(ball) ↓ form an equilibrium pair — not an interaction pair.

E4 · Evaluation Questions

Evaluation

Six questions testing specific aspects of the distinction. Read carefully — each is designed to expose a common error.

Question 1 · Interaction pairs
A man pushes a wall with force F. The wall pushes back on the man with F′ = −F. These two forces cannot cancel each other because:
AThey are not equal in magnitude
BThey act on different bodies — F on the wall, F′ on the man
CThey act in the same direction
DOne is contact, one is distance force
Question 2 · Equilibrium pair identification
A lamp hangs from a ceiling by a wire. Tension T acts upward on the lamp. Weight W acts downward on the lamp. These form:
AAn interaction pair — T is the reaction to W
BAn equilibrium pair — both act on the lamp, making it stationary
CNeither — they are not related to each other
DAn interaction pair — any equal and opposite forces form an interaction pair
Question 3 · The Classic Trap
A student says: "W and N on a book are an interaction pair because N = −W." What is wrong?
A|N| ≠ |W| so they cannot be paired
BBoth N and W act on the book → equilibrium pair. The true interaction partner of N is F(book→table) which acts on the TABLE
CInteraction pairs must involve only contact forces
DN and W are not truly equal and opposite
Question 4 · Deduction
A ball of weight W = 8 N (downward) floats on water. Using the equilibrium condition, deduce Fb (buoyancy). Then identify the interaction partner of Fb.
AFb = 8 N downward · interaction partner = W
BFb = 8 N upward · interaction partner = force ball exerts on water downward (8 N)
CFb = 4 N upward · balances half of W
DFb = 8 N upward · interaction partner = W (weight of ball)
Question 5 · True or False
"An interaction pair can produce equilibrium of a single body."
ATrue — the forces are equal and opposite so they cancel
BFalse — interaction pairs act on different bodies so they can NEVER cancel each other or produce equilibrium of one body
Question 6 · Synthesis (Written)
A body of weight W = 15 N hangs from a spring. (a) State all forces on the body. (b) Apply the equilibrium condition to find the spring force Fs. (c) Identify the interaction partner of Fs and state which body it acts on.
Written answer saved with your submission.
E5 · Submit Report

Submit Your Report

Choose your section, select your name, then submit everything to Ahmad's grade sheet.

0 / 9 activities completed

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Score

Ahmad reviews your written answers and Verdict Chamber in class.