How Forces Affect Motion – Class 9 Science Chapter 6 Notes & NCERT PDF

Class 9ScienceChapter 6NCERT book: Exploration - Science Class 9

Notes and a simple summary of Chapter 6 of the NCERT Class 9 Science book, with the official chapter PDF, flashcards, an MCQ quiz and an AI tutor for your doubts.

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Chapter 6: How Forces Affect Motion

Think It Over

Why does a canoe move forward when the canoeist pushes water backwards with the paddle, and why does it go faster when they push harder? If the same paddle force is used on an empty canoe and on one carrying a passenger, which moves faster?

Introduction

Chapter 4 described motion (position, velocity, acceleration). This chapter asks what causes changes in motion — the answer is force — and studies Newton's three laws of motion and how to apply them.

Key Concepts

1. The concept of force

A force can start motion, change speed or direction, or change shape (kicking a ball, a bat striking a ball, squeezing a lemon).

Force needs magnitude and direction (like displacement and velocity). SI unit: newton (N) — the unit named after Newton is written with a small 'n', its symbol with capital N.

A spring balance measures weight (the Earth's gravitational pull) and forces in general.

Everyday forces we can feel are around millinewtons (10⁻³ N); scientists have measured forces as small as yoctonewtons (10⁻²⁴ N) (as of 2026).

2. Balanced and unbalanced forces

Equal and opposite forces on an object are balanced (tug of war with equal pulls — the rope does not move).

If forces are unbalanced, a net force acts:

- Opposite directions → net force = difference, in the direction of the larger force.

- Same direction → net force = sum (two people pushing a stalled car).

Book example 6.1: 10 N and 6 N on a block: same direction → 16 N; opposite → 4 N towards the larger force.

An object's motion depends only on the net force.

3. The force of friction

Friction acts opposite to the direction of motion (or attempted motion) between surfaces.

A box moves only when your push exceeds friction; when you stop pushing, friction brings it to rest.

Activity 6.1: a stack of four ₹10 coins launched by a stretched rubber band travels farthest on polished marble/tile, less on laminated top, least on wood — less friction, slower loss of velocity, longer distance.

Activity 6.2: a spring balance pulling a wooden block gives a rough measure of friction on each surface.

Thought experiment (Galileo, 17th century): on a perfectly frictionless floor, a moving object would never stop. Newton used the word inertia for an object's tendency to resist change in its state of rest or uniform motion, and published his three laws in 1687.

4. Newton's first law of motion

An object at rest remains at rest, and an object in motion continues to move with a constant velocity, unless a net force acts upon the object.

Zero net force → zero acceleration (either at rest, or constant velocity in a straight line).

Example 6.2: pushing a moving box with a force exactly equal to friction → net force zero → box moves at constant velocity.

5. Newton's second law of motion

When a net force acts on an object, the object accelerates in the direction of the net force. The magnitude of the acceleration is proportional to the net force and inversely proportional to the mass.

a = F ÷ m, or F = ma

Activities 6.3 and 6.4: a cardboard-box cart pulled by a cup of coins over a pipe-pulley. Doubling the hanging mass (force) increases acceleration; doubling the cart's mass decreases it. Using s = ½aT² for the same distance: a₁T₁² = a₂T₂². Friction makes real results slightly off from exact doubling/halving.

1 N = force giving 1 kg an acceleration of 1 m s⁻². Holding a 100 g mass in your palm needs about 1 N.

Weight: F = mg, with g = 9.8 m s⁻² (≈ 10 for estimates). g does not depend on the object's mass.

Momentum = mass × velocity; the fuller form of the second law: rate of change of momentum ∝ net force.

Everyday uses: a fielder pulls hands back while catching (more time → smaller acceleration → smaller force); airbags and bubble wrap/hay do the same; a coconut hitting the ground stops in a very short time, so a very large force cracks it.

6. Newton's third law of motion

Whenever one object exerts a force on a second object, the second object simultaneously exerts an equal and opposite force on the first.

Activity 6.5: on a wheeled chair, pushing a heavy table moves you backwards; pulling it moves you forwards.

Activity 6.6: two connected spring balances pulled apart always read the same.

Walking/running: your foot pushes the ground back; friction pushes you forward (so friction helps here — grooves on soles, treads on tyres).

Coconut-tree climbing, rowing a canoe, the balloon on a thread (Activity 6.7), and rockets — exhaust pushed down, rocket pushed up. Vikram lander of Chandrayaan-3 fired its engines in the direction of motion to slow down for a soft landing near the Moon's south pole.

The two forces of a pair act on different objects, so they do not cancel. The law applies to contact and non-contact forces (magnets, charged balloons, the Earth and a falling fruit). Equal forces can give unequal accelerations because masses differ.

7. Forces on a system of objects

Two boxes (m₁, m₂) joined by a string on a frictionless floor, pulled by F: the string's tension T is an internal force; treat both as one system: a = F ÷ (m₁ + m₂).

Treating connected objects as one system simplifies problems — like studying your whole body while walking.

Worked Numericals

Barbell (Example 6.4): 10 kg + 10 kg + 10 kg bar = 30 kg; F = mg = 30 × 9.8 = 294 N upward to hold it steady.

Pushing a block (Example 6.5): 25 kg block, friction 50 N. Push 50 N → balanced, no motion. Push 55 N → net 5 N, a = 5/25 = 0.2 m s⁻², s = ½ × 0.2 × 2² = 0.4 m in 2 s.

Sports car (Example 6.6): 1500 kg, 0 → 10 m s⁻¹ in 5 s → a = 2 m s⁻², F = 3000 N east; constant velocity → no net force; 10 → 0 in 5 s → F = –3000 N (west).

Gun and bullet (Example 6.8): force 2 N on each; gun (5 kg) a = 0.4 m s⁻²; bullet (0.1 kg) a = 20 m s⁻².

Snake boat (exercise): 95 row one way, 5 the other, 200 N each → net = 90 × 200 = 18 000 N.

Bullet in wood (exercise): 50 g at 100 m s⁻¹ stops in 0.5 m → a = 100²/(2 × 0.5) = 10 000 m s⁻² → F = 0.05 × 10 000 = 500 N.

Penalty kick (exercise): 108 km h⁻¹ = 30 m s⁻¹, F = 800 N, m = 0.4 kg → a = 2000 m s⁻² → contact time = 30/2000 = 0.015 s.

Rough patch (exercise): 2 kg at 10 m s⁻¹, opposing forces 7 N + 3 N = 10 N → a = –5 m s⁻² → s = 10²/(2 × 5) = 10 m.

Activities in the Chapter

6.1 Coin stack and rubber band on different surfaces.

6.2 Measuring friction with a spring balance and wooden block.

6.3 / 6.4 Cart-and-pulley experiments: varying force, then varying mass.

6.5 Wheeled chair pushing and pulling a table.

6.6 Two connected spring balances.

6.7 Balloon rocket along a thread.

Key Terms

Force (बल)

Net force (परिणामी / नेट बल)

Balanced forces (संतुलित बल)

Unbalanced forces (असंतुलित बल)

Friction (घर्षण)

Inertia (जड़त्व)

Mass (द्रव्यमान)

Weight (भार)

Momentum (संवेग)

Action–reaction pair (क्रिया–प्रतिक्रिया युग्म)

Tension (तनाव)

Spring balance (कमानीदार तुला)

System (निकाय)

Newton (unit) (न्यूटन)

Real-life Connections

Airbags, seat belts, landing mats and sand pits increase stopping time to reduce force.

Grooved soles and tyre treads increase friction so we can walk and vehicles can grip roads; wet floors and ice are slippery.

Rockets and Chandrayaan-3's Vikram lander use the third law.

Fire hoses push back on firefighters as water rushes out.

Common Misconceptions

"A force is needed to keep an object moving." — Only to overcome friction; without friction a moving object keeps moving (first law).

"Action and reaction cancel each other." — They act on different objects, so they never cancel.

"Equal forces mean equal accelerations." — Acceleration also depends on mass (gun vs bullet; Earth vs fruit).

"Constant velocity means a net force is acting." — Constant velocity means zero net force.

"Heavier objects fall with greater acceleration." — g does not depend on mass.

Writing the unit as "Newton" or symbol "n" — the unit is newton, symbol N.

Revise, Reflect, Refine — Hints

Draw a force diagram and add forces with signs before using F = ma.

If an object moves at constant velocity, the applied force equals friction.

For time-of-contact and stopping-force problems, find acceleration from kinematics first, then use F = ma.

Tractor question: m₁ = F/a₁, m₂ = F/a₂, so combined acceleration = F ÷ (m₁ + m₂) = a₁a₂ ÷ (a₁ + a₂).

💡 Key Learning Points

✓Force has magnitude and direction; motion depends on the net force from balanced or unbalanced forces.
✓Friction opposes motion; with no friction a moving object keeps moving (Galileo, inertia, first law).
✓Apply Newton's second law F = ma and weight F = mg, including stopping-force and contact-time problems.
✓Explain Newton's third law with walking, rowing, rockets and Chandrayaan-3, and why action–reaction pairs do not cancel.
✓Treat connected objects as a single system: a = F ÷ (m₁ + m₂).

👨‍🏫 Teaching Tips

→Run Activity 6.1 with a stack of four ₹10 coins and a rubber band on several classroom surfaces; tabulate distances.
→Build the cardboard cart and paper-cup pulley (Activities 6.3/6.4) and record slow-motion video on a phone to time the runs.
→Let students sit on a wheeled chair and push/pull a heavy table (Activity 6.5) to feel the third law.
→Launch a balloon rocket along a string across the room, then show a clip of a rocket or Chandrayaan-3 landing.
→Use a raw egg dropped onto a cushion vs a hard floor (in a bag) to discuss stopping time and force.
→Emphasise 'forces on the same object balance; action–reaction act on different objects' with labelled force diagrams.
→Solve every numerical by first drawing a force diagram and finding the net force before using F = ma.

📋 Assessment Questions

Can the student find the net force for forces in the same and opposite directions?
Can the student state all three laws and identify which law explains a given everyday situation?
Can the student solve F = ma problems combined with kinematic equations (stopping force, contact time, distance)?
Can the student explain why action–reaction forces do not cancel and why equal forces give unequal accelerations?
Can the student apply a = F ÷ (m₁ + m₂) to a system of connected objects?

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