Grade 9 · Physics · Forces Part 2

Forces in Equilibrium

A body at rest is held perfectly still by forces that cancel each other out. Discover the condition for that balance — then explore what happens when two bodies interact.

🤔 A book rests motionless on a table. Gravity pulls it down — yet it does not fall. A spring stretches and holds still. A ball floats without sinking.

What is the exact condition that keeps each body perfectly still?

E2 · Explore

Place the Force Arrows

For each diagram, tap a force button to select it, then tap on the figure to place the arrow at the correct point. Weight starts from G (centre of gravity). Contact forces start from the exact contact point.

A — Book on Table
Click to place arrows
Tap a force below, then tap the diagram
The normal reaction N is a:
AContact force
BForce at a distance
B — Ball Floating on Water
Click to place arrows
Tap a force below, then tap the diagram
Buoyancy is a:
AContact force (water touches ball)
BForce at a distance
C — Body Suspended from Spring
Click to place arrows
Tap a force below, then tap the diagram
Weight is a:
AContact force
BForce at a distance (gravity)
E3 · Equilibrium Condition

Condition of Equilibrium

A body acted on by two forces is in equilibrium when:

F1 = −F2
Equal magnitudes · Opposite directions · Same line of action
|F1| = |F2|
The two forces are equally strong
Opposite directions
One up, one down — or one left, one right
Same line of action
Both forces along the same straight line
Three Examples with Complete Diagrams
Example 1
Book Resting on a Horizontal Table
Forces on the book:
• Weight W — gravitational force, origin: centre of gravity G, direction: vertically downward, type: distance force
• Normal reaction N — from table surface, origin: contact point (bottom face of book), direction: vertically upward, type: contact force
N = −W
N: vertical ↑ from contact point · W: vertical ↓ from G · |N| = |W|
If W = 5 N downward, deduce N:
AN = 5 N, vertically downward
BN = 5 N, vertically upward
CN = 10 N, vertically upward
Example 2
Ball Floating on the Surface of Water
Forces on the ball:
• Weight W — origin: centre of ball G, direction: vertically downward
• Buoyancy Fb — from water, origin: lowest contact point of ball with water, direction: vertically upward
Fb = −W
Fb: vertical ↑ from contact · W: vertical ↓ from G · |Fb| = |W|
Buoyancy is exerted by the water. It is a:
AContact force — water touches the ball directly
BForce at a distance — like gravity
Example 3
Body Suspended from a Spring
Forces on the body:
• Weight W — origin: centre of gravity G, direction: vertically downward
• Tension T — from spring, origin: top contact point of body with spring, direction: vertically upward
T = −W
T: vertical ↑ from contact point · W: vertical ↓ from G · |T| = |W|
If W = 12 N downward, deduce T:
AT = 12 N, vertically downward
BT = 6 N, vertically upward
CT = 12 N, vertically upward
E3 · Principle of Interaction

Principle of Interaction

Statement

When body A exerts a force on body B, at the same moment, body B exerts a force on body A.

FA/B = −FB/A
|FA/B| = |FB/A| — equal magnitudes, opposite directions, different bodies
⚠ Key: These forces act on different bodies — they do NOT cancel each other.
Box / Table: Ftable/box (table pushes box ↑) = −Fbox/table (box pushes table ↓)
Ball / Water: Fwater/ball (water pushes ball ↑) = −Fball/water (ball pushes water ↓)
Body / Spring: Fspring/body (spring pulls body ↑) = −Fbody/spring (body pulls spring ↓)
🎬 Live Demo — Person Pushing a Wall
Each bounce: force decreases as the ball loses energy!
F(ball/floor) — ball pushes floor ↓
F(floor/ball) — floor pushes ball ↑ (reaction)
When the ball hits the floor harder, the reaction force from the floor:
AStays the same — it doesn't change
BIncreases by the same amount — always equal in magnitude
CDecreases — the wall resists less
E4 · Practice

Apply What You Know

Question 1 · Deduce Force Characteristics
A ball floats on water. Its weight is W = 8 N, vertically downward. Deduce all characteristics of the buoyancy force Fb.
AFb = 8 N, vertically downward, vertical line of action
BFb = 4 N, vertically upward, vertical line of action
CFb = 8 N, vertically upward, vertical line of action
DFb = 8 N, horizontally, horizontal line of action
Question 2 · Deduce Force Characteristics
A body hangs from a spring at rest. Tension T = 15 N, vertically upward. What are the characteristics of the weight W?
AW = 15 N, vertically downward, vertical line of action
BW = 15 N, vertically upward, vertical line of action
CW = 7.5 N, vertically downward
Question 3 · Interaction Pair
A box rests on a table. The table exerts a force on the box of 20 N upward. Identify the reaction force — who exerts it, on whom, and what are its characteristics?
AEarth exerts 20 N downward on the box
BBox exerts 20 N downward on the table
CBox exerts 20 N upward on the table
⚠ Critical Thinking
Question 4 · Common Misconception
A student says: "The weight of the box and the normal reaction of the table form an interaction pair." Is this correct?
AYes — they are equal, opposite, and on the same line
BNo — they both act on the same body (the box), so they cannot be an interaction pair
CNo — they are not equal in magnitude
Question 5 · Formula Builder + AI Tutor
A body of weight W = 10 N rests on a horizontal table.
(a) Name all forces and classify each.
(b) Build the equilibrium condition by tapping the tiles below.
(c) Deduce all characteristics of normal reaction N.
(d) Identify and build the interaction pair vector relation.
Part (b) — Tap tiles to build the equilibrium condition:
Your formula appears here...
Part (d) — Build the interaction pair relation:
Your interaction formula appears here...
Parts (a) and (c) — write your answers:
E5 · Submit

Submit Your Work

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Ahmad reviews your force diagrams and open answer in the next class.