💥 Class 9 Science – How Forces Affect Motion Original Visual Revision Notes
Aligned with the Class 9 Science syllabus. Independently created study material. Not affiliated with or endorsed by NCERT.
Why does a football fly through the air when you kick it? Why does a car slow down when you press the brakes? Why does a rocket blast off into space? The answer to all these questions is a single word: force. A force is simply a push or a pull — and yet forces are responsible for everything that moves, stops, bends, or breaks in the universe!
Imagine you are pushing a heavy box across the floor. If you push harder, it moves faster. If there is less friction under the box, it slides more easily. If another person pushes from the other side at the same time, the box might not move at all! These everyday experiences perfectly illustrate the ideas that Isaac Newton organised into three brilliant laws of motion, which changed science forever.
In this chapter, we explore Newton's Three Laws of Motion, understand what momentum is and why it is conserved, and learn about friction — the force that makes life both easier and harder. By the end, you will understand why seat belts save lives, why rockets work in the vacuum of space, and how a goalkeeper uses force to stop a penalty shot!
💪 Topic 1: What is Force
A force is a push or a pull.
Force can make things start moving.
Force can stop a moving thing.
Force can change the shape of things.
The SI unit of force is newton (N).
📏 Topic 2: Measuring Force
We use a spring balance to measure force.
The spring stretches more with more force.
Force has both size and direction.
We read the scale on the spring balance.
The reading shows the force in newtons.
⚖ Topic 3: Balanced and Unbalanced Forces
Equal forces in opposite ways are balanced.
Balanced forces do not change motion.
Unbalanced forces make things speed up or slow.
Net force is the total of all forces.
Only unbalanced forces cause acceleration.
👣 Topic 4: Force of Friction
Friction acts against the direction of motion.
Rough surfaces have more friction.
Smooth surfaces have less friction.
Friction slows down moving things.
We need force to overcome friction.
🚶 Topic 5: Newton's First Law – Inertia
A still thing stays still unless pushed.
A moving thing keeps moving unless stopped.
This tendency is called inertia.
More mass means more inertia.
This is Newton's first law of motion.
⏰ Topic 6: Understanding Inertia
Inertia depends on mass of the object.
A heavy ball is harder to push than a light one.
A moving bus stops but passengers keep going.
Seat belts protect us because of inertia.
Inertia keeps things doing what they do.
🚀 Topic 7: Newton's Second Law – F = ma
Force equals mass times acceleration.
F = m × a (written as F = ma).
More force gives more acceleration.
More mass gives less acceleration.
The unit of force is kg·m/s² = 1 newton.
🔄 Topic 8: Newton's Third Law
Every action has an equal opposite reaction.
When you push a wall, it pushes you back.
A rocket pushes gas down and goes up.
Action and reaction act on different objects.
Both forces are equal in size.
🎯 Topic 9: Conservation of Momentum
Momentum = mass × velocity (p = mv).
Total momentum before = total momentum after.
This is the law of conservation of momentum.
It works when no outside force acts.
Momentum is measured in kg·m/s.
📐 Topic 10: Applications of Newton's Laws
Seat belts use the first law of motion.
Rockets use the third law to fly up.
Athletes push the ground to jump high.
Heavy trucks need more force to stop.
Friction helps us walk without slipping.
🎨 Section 2 — Easy Illustrations
Fig 1: Balanced forces vs unbalanced forces
Fig 2: Four forces act on a box on the ground
Fig 3: More force on the same mass gives more acceleration
Fig 4: Every push has an equal push back
Fig 5: Total momentum stays the same after collision
✏️ Section 3 — Worked Examples
💡 Example 1: Net Force
Two children push a cart. One pushes with 15 N to the right. The other pushes with 9 N to the left. Find the net force.
1
The two forces act in opposite ways.
2
Net force = 15 − 9 = 6 N to the right.
Net Force = Bigger Force − Smaller Force
Answer: The net force is 6 N to the right.
💡 Example 2: Newton's Second Law (F = ma)
A box has a mass of 8 kg. A net force of 24 N acts on it. Find the acceleration.
1
We know F = m × a.
2
So a = F ÷ m = 24 ÷ 8 = 3 m/s².
a = F ÷ m
Answer: The acceleration is 3 m/s².
💡 Example 3: Conservation of Momentum
A ball of mass 2 kg moves at 5 m/s. It hits a still ball of mass 3 kg. They stick together. Find their speed.
1
Momentum before = 2 × 5 + 3 × 0 = 10 kg·m/s.
2
After: (2+3) × v = 10, so v = 10 ÷ 5 = 2 m/s.
m₁v₁ + m₂v₂ = (m₁ + m₂) × v
Answer: They move together at 2 m/s.
📋 Section 4 — Cheat Sheet
Term
Meaning
Force
A push or a pull on an object.
Newton (N)
The SI unit for force.
Net Force
Total force after adding all forces.
Balanced Forces
Equal forces that cancel out.
Unbalanced Forces
Forces that do not cancel out.
Friction
A force that opposes motion.
Inertia
Tendency to resist change in motion.
Mass
Amount of matter in an object.
Acceleration
Rate of change of velocity.
Momentum
Mass times velocity of an object.
Newton's First Law
Objects resist change unless a force acts.
Newton's Second Law
Force equals mass times acceleration.
Newton's Third Law
Every action has an equal reaction.
Conservation of Momentum
Total momentum stays the same.
Action-Reaction
A pair of equal, opposite forces.
Spring Balance
A tool used to measure force.
🏠 Section 5 — Take-Home Handout
Part A: Fill in the Blanks
1. A force is a push or a _______.
2. The SI unit of force is the _______.
3. Balanced forces do not change _______.
4. A moving thing keeps moving due to _______.
5. Momentum equals mass times _______.
Part B: True or False
1. Friction always helps motion.
2. A heavy object has more inertia.
3. Unbalanced forces cause acceleration.
4. Newton's third law needs two objects.
5. Momentum is never conserved.
Part C: Solve
1. A cart has 18 N to the right and 10 N to the left. Find the net force.
2. A toy car has mass 4 kg. A force of 20 N pushes it. Find its acceleration.
Part D: Match the Columns
Column A
Column B
Your Answer
1. Newton's First Law
A. Mass × Velocity
2. Newton's Second Law
B. Action equals Reaction
3. Newton's Third Law
C. Opposes Motion
4. Friction
D. F = m × a
5. Momentum
E. Law of Inertia
Part E: Draw and Label
Draw force arrows on the box below. Label each arrow.
A force is a push or a pull. Forces cause objects to start, stop, or change direction.
The SI unit of force is the newton (N). One newton = the force needed to accelerate 1 kg at 1 m/s².
Balanced forces do not change motion (velocity). The object stays at rest or continues at constant velocity.
A moving thing keeps moving due to inertia — its tendency to resist change in state of motion (Newton’s First Law).
Momentum equals mass times velocity. Formula: p = m × v. Unit is kg·m/s.
Part B — True or False (Answers)
False. Friction opposes (resists) motion. It acts in the opposite direction to movement. Sometimes friction is useful (gripping), but it does not help motion — it hinders it.
True. A heavier object has more inertia — it takes more force to start or stop it. This is why it is harder to push a full trolley than an empty one.
True. Unbalanced forces (net force ≠ 0) cause acceleration — a change in speed or direction (Newton’s Second Law: F = ma).
True. Newton’s Third Law always involves two objects — one exerts the action force, the other exerts the equal and opposite reaction force.
False. Momentum IS conserved in a closed system (no external forces). The total momentum before a collision equals the total momentum after.
Part C — Solve (Step-by-Step)
C1. Cart: 18 N right, 10 N left. Find net force.
1Net force = Force right − Force left (taking right as positive)
2= 18 − 10 = 8 N
✔ Net force = 8 N to the right. The cart accelerates to the right.
C2. Toy car: mass 4 kg, force 20 N. Find acceleration.
1Newton’s Second Law: F = m × a
2Rearrange: a = F ÷ m = 20 ÷ 4
3= 5
✔ Acceleration = 5 m/s²
Part D — Match the Columns (Answers)
Newton’s First Law → E. Law of Inertia — Objects resist change; at rest stays at rest, moving stays moving.
Newton’s Second Law → D. F = m × a — Force equals mass times acceleration.
Newton’s Third Law → B. Action equals Reaction — Every force has an equal and opposite force pair.
Friction → C. Opposes Motion — Acts opposite to direction of movement.
Momentum → A. Mass × Velocity — p = mv; conserved in closed systems.
Part E — Force Arrow Diagram (Guidance)
Label these forces on the box diagram:
Applied Force (→): Arrow pointing to the right — the push you apply to the box.
Friction (←): Arrow pointing to the left — opposes the applied force.
Normal Force (↑): Arrow pointing upward — the surface pushing up on the box.
If Applied Force > Friction, the box accelerates. If they are equal, the box is in equilibrium.
🃏 Section 6 — Revision Flashcards
Light side = Question. Dark side = Answer.
What is a force?
A push or a pull.
What is the unit of force?
Newton (N).
What are balanced forces?
Equal forces in opposite ways.
What causes acceleration?
An unbalanced force (net force).
What is inertia?
Tendency to resist change in motion.
What is Newton's second law?
F = m × a.
What is Newton's third law?
Every action has an equal reaction.
What is momentum?
Mass × velocity (p = mv).
🧠 Section 7 — Mind Map
Fig 6: Mind map of how forces affect motion
⚠ This is independently created study material aligned with the Class 9 Science syllabus. Not affiliated with or endorsed by NCERT or any government body.