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Rajasthan (RBSE)Class 9 Science★ 7–9 marks · RBSE Class 9 Science (new NCF — Curiosity)

How Forces Affect Motion

The new NCF Class 9 Science chapter on force and Newton's laws. Balanced vs unbalanced forces, friction, the three laws of motion, inertia, momentum, force = ma, action–reaction pairs, and conservation of momentum — with fully solved board-pattern numericals.

⏱ 36 min readHard difficulty✓ Free · NCERT-aligned

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The chapter at a glance

Every concept and exam-favourite branch — revisable in 5 minutes.

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Study Guide

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Key points, glossary and the questions most likely to show up.

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By the end of this chapter you'll be able to…

1Define force and list its effects; distinguish balanced from unbalanced forces
2Explain friction as a force opposing relative motion
3State and apply Newton's first law and relate inertia to mass
4Define momentum (p = mv) and apply Newton's second law F = ma
5State Newton's third law and identify action–reaction pairs on two bodies
6Apply conservation of momentum to simple collisions and recoil

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Why this chapter matters

Newton's laws are the heart of mechanics. F = ma and conservation of momentum are reused in Class 11 Laws of Motion and throughout JEE/NEET physics. The chapter blends conceptual reasoning (inertia, action–reaction) with numericals, so it rewards students who can both explain and calculate.

Before you start — revise these

A 5-minute refresher here will save you 30 minutes of confusion below.

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Describing Motion Around Us (Class 9)

Velocity and acceleration — directly used in F = ma and momentum.

How Forces Affect Motion (RBSE Class 9 · Science)

In the last chapter you learned to describe motion. Now you ask why motion changes at all. The answer is a single idea — force — and three short laws written by Newton that still steer rockets and cricket balls alike.

RBSE note (2026-27). Class 9 uses the new NCF (Curiosity) Science textbook. This chapter, How Forces Affect Motion, follows Describing Motion Around Us and develops Newton's laws and momentum. BSER (Ajmer) sets the exam.

1. What is a force?

A force is a push or a pull. It cannot always be seen, but its effects can:

it can start a stationary object moving, or stop a moving one;
it can change the speed or the direction of motion;
it can change the shape of an object.

Force is a vector (magnitude + direction). Its SI unit is the newton (N).

2. Balanced and unbalanced forces

When several forces act on an object:

Balanced forces add up to zero net force → no change in the state of motion (the object stays at rest or keeps moving uniformly). A book on a table: gravity down = normal force up.
Unbalanced forces give a non-zero net force → the object accelerates (speeds up, slows down or changes direction).

Only an unbalanced force can change the state of motion of an object.

3. Friction

Friction is the force that opposes relative motion between two surfaces in contact. It acts opposite to the direction of motion. It is why a ball rolling on the ground eventually stops, and why we can walk without slipping. Friction can be reduced (lubricants, ball bearings, streamlining) or increased (treads, grooves) as needed.

4. Newton's First Law — the law of inertia

An object continues in its state of rest or of uniform motion in a straight line unless acted upon by an unbalanced force.

This natural tendency to resist a change in the state of motion is inertia. More mass ⇒ more inertia (a loaded truck is harder to start or stop than a bicycle). Everyday inertia: passengers lurch forward when a bus brakes; dust flies off a beaten carpet.

5. Momentum and Newton's Second Law

Momentum (p) measures the "quantity of motion": the product of mass and velocity.

$p = m v$

Unit: kg·m/s; it is a vector. Newton's second law:

The rate of change of momentum is directly proportional to the applied force, and is in the direction of the force.

This leads to the most-used equation in mechanics:

$F = ma$

A force of 1 newton gives a 1 kg mass an acceleration of 1 m/s² (1 N = 1 kg·m/s²). The second law explains why we follow through in cricket and bend our knees on landing — increasing the time of contact reduces the force for the same change in momentum.

6. Newton's Third Law

To every action there is an equal and opposite reaction.

Action and reaction are equal in magnitude, opposite in direction, and act on two different objects (so they do not cancel). A swimmer pushes water back; the water pushes the swimmer forward. A gun recoils when a bullet is fired.

7. Conservation of momentum

For two objects interacting (e.g. a collision) with no external unbalanced force, the total momentum before = total momentum after:

$m_{1} u_{1} + m_{2} u_{2} = m_{1} v_{1} + m_{2} v_{2}$

This is the law of conservation of momentum — the principle behind rocket propulsion and recoil.

8. Worked example

A force of 10 N acts on a 2 kg object at rest for 3 s. Find (a) the acceleration, (b) the final velocity, and (c) the momentum gained.

(a) a = F/m = 10/2 = 5 m/s².

(b) v = u + at = 0 + 5 × 3 = 15 m/s.

9. Quick recap

A force (newton) is a push/pull; it can change motion, direction or shape.
Balanced forces → no change; unbalanced force → acceleration.
Friction opposes relative motion.
1st law (inertia): objects resist changes in motion; inertia ∝ mass.
Momentum p = mv (kg·m/s); 2nd law F = ma (1 N = 1 kg·m/s²).
3rd law: equal and opposite reactions on two different bodies.
Conservation of momentum: total momentum is unchanged with no external force.

Key formulas & results

Everything you need to memorise, in one card. Screenshot this for revision.

Momentum

p = mv

Vector; unit kg·m/s.

Newton's second law

F = ma

1 N = 1 kg·m/s².

Force from momentum

F = (mv − mu) / t

Rate of change of momentum.

Conservation of momentum

m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂

No external unbalanced force.

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Common mistakes & fixes

These are the exact errors that cost students marks in board exams. Read them once, save yourself the trouble.

WATCH OUT

✗ Saying action and reaction cancel out

✓ They act on TWO DIFFERENT objects, so they never cancel on a single body. Cancellation needs both forces on the same object.

WATCH OUT

✗ Thinking a moving object needs a continuous force to keep moving

✓ By the first law, no force is needed to maintain uniform motion; force is needed only to CHANGE motion. (Real objects slow down because of friction.)

WATCH OUT

✗ Confusing mass and inertia with weight

✓ Inertia depends on mass (kg). Weight is the gravitational force (N) and varies with g; mass does not.

WATCH OUT

✗ Forgetting momentum is a vector

✓ Assign directions (+/−) before adding momenta in collision problems, or the answer's sign/size is wrong.

WATCH OUT

✗ Using F = ma with mismatched units

✓ Mass in kg, acceleration in m/s² → force in newtons. Convert grams/cm first.

NCERT exercises (with solutions)

Every NCERT exercise from this chapter — what it covers and how many questions to expect.

In-text (force & friction)

Effects of force, balanced/unbalanced, friction

Questions

In-text (laws of motion)

Inertia, F = ma, action–reaction

Questions

Activity

Inertia demonstrations; balloon-rocket for the third law

Questions

End-of-chapter Ex

End-of-chapter exercise

Concept + numericals on F = ma and momentum

Questions

Practice problems

Try each one yourself before tapping "Show solution". Active recall > rereading.

Q1EASY· Force

State the SI unit of force and define 1 newton.

▶ Show solution

The SI unit is the newton (N). 1 N is the force that gives a 1 kg mass an acceleration of 1 m/s² (1 N = 1 kg·m/s²).

✦ Answer: newton; 1 N = 1 kg·m/s².

Q2EASY· Inertia

Why do passengers lurch forward when a moving bus suddenly stops?

▶ Show solution

By inertia, the body tends to continue in its state of motion; when the bus stops, the upper body keeps moving forward.

✦ Answer: due to inertia of motion.

Q3EASY· Momentum

Find the momentum of a 4 kg ball moving at 5 m/s.

▶ Show solution

Step 1 — p = mv = 4 × 5.

Step 2 — p = 20 kg·m/s.

✦ Answer: 20 kg·m/s.

Q4MEDIUM· F = ma

What force gives a 5 kg body an acceleration of 3 m/s²?

▶ Show solution

Step 1 — F = ma = 5 × 3.

Step 2 — F = 15 N.

✦ Answer: 15 N.

Q5MEDIUM· Third law

Explain, using Newton's third law, how a swimmer moves forward.

▶ Show solution

Step 1 — The swimmer pushes the water backward (action).

Step 2 — The water pushes the swimmer forward with an equal and opposite force (reaction).

Step 3 — The forces act on different bodies, so the swimmer accelerates forward.

✦ Answer: action on water backward → reaction on swimmer forward.

Q6MEDIUM· Change in momentum

A 0.5 kg ball's velocity changes from 4 m/s to 10 m/s in 2 s. Find the force.

▶ Show solution

Step 1 — F = (mv − mu)/t = (0.5×10 − 0.5×4)/2.

Step 2 — F = (5 − 2)/2 = 3/2 = 1.5 N.

✦ Answer: 1.5 N.

Q7HARD· Conservation

A 2 kg trolley moving at 3 m/s collides with a stationary 1 kg trolley and they move off together. Find their common velocity.

▶ Show solution

Step 1 — Total momentum before = 2×3 + 1×0 = 6 kg·m/s.

Step 2 — After: combined mass = 3 kg moving at v.

Step 3 — 6 = 3v → v = 2 m/s.

✦ Answer: 2 m/s.

Q8HARD· Recoil

A 4 kg gun fires a 0.02 kg bullet at 100 m/s. Find the recoil velocity of the gun.

▶ Show solution

Step 1 — Initial momentum = 0 (both at rest).

Step 2 — 0 = (0.02 × 100) + (4 × V).

Step 3 — 0 = 2 + 4V → V = −0.5 m/s.

✦ Answer: 0.5 m/s, opposite to the bullet (recoil).

Q9HARD· Reasoning

A cricketer pulls his hands back while catching a fast ball. Explain using momentum and force why this prevents injury.

▶ Show solution

Step 1 — The ball has a fixed momentum that must be reduced to zero (a fixed change in momentum).

Step 2 — By the second law, F = change in momentum / time.

Step 3 — Pulling the hands back INCREASES the time over which the ball stops.

Step 4 — A larger time means a smaller force on the hands, so injury is avoided.

✦ Answer: longer contact time → smaller force (F = Δp/t).

5-minute revision

The whole chapter, distilled. Read this the night before the exam.

•Force (newton) is a push/pull; changes speed, direction or shape; it is a vector.
•Balanced forces (net 0) → no change; unbalanced force → acceleration.
•Friction opposes relative motion between surfaces in contact.
•First law = inertia: objects resist changes in motion; inertia increases with mass.
•Momentum p = mv (kg·m/s); Newton's second law F = ma (1 N = 1 kg·m/s²).
•Third law: equal and opposite reactions act on two DIFFERENT bodies (don't cancel).
•Conservation of momentum: m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂ with no external force.
•F = Δp/t explains follow-through, bending knees on landing and pulling hands back to catch.

Rajasthan (RBSE) marks blueprint

Where the marks come from in this chapter — so you can plan your prep.

Typical chapter weightage: 7–9 marks

Question type	Marks each	Typical count	What it tests
MCQ / Assertion–Reason	1	1–2	Units, inertia, action–reaction statements
Short answer / numerical	2	2	F = ma, momentum, third-law examples
Short answer	3	1	Conservation of momentum; recoil
Long / numerical + reasoning	4–5	1	Collision problem; force–time reasoning

Prep strategy

Learn the three laws verbatim — statements are direct 1–2 mark questions
Practise F = ma and p = mv until automatic; mind the units
For collisions, write momentum before = momentum after with signs
Prepare two crisp real-life examples for each law

Where this shows up in the real world

This chapter isn't just an exam topic — it lives in the world around you.

Seatbelts and airbags

They lengthen the time to stop you in a crash, cutting the force (F = Δp/t) — Newton's second law saving lives.

Rockets

Hot gases pushed down (action); the rocket is pushed up (reaction) — the third law and conservation of momentum.

Walking

You push the ground backward; the ground pushes you forward. No friction, no walking.

Cricket catch / high-jump landing

Pull hands back or bend knees to extend contact time and reduce the force.

Exam strategy

Battle-tested tips from teachers and toppers for this chapter.

Quote the law's statement first, then give the example or numerical — both earn marks.
For numericals, write F = ma or p = mv, substitute with units, then solve.
In collisions, fix a positive direction and keep momentum signs consistent.
For 'why' questions, link to F = Δp/t or inertia explicitly.
Always end with the unit (N, kg·m/s) on the final answer.

Going beyond the textbook

For olympiad aspirants and curious learners — topics that build on this chapter.

Impulse (J = FΔt = Δp) and the impulse–momentum theorem.
Friction in detail: static vs kinetic, coefficient of friction.
Elastic vs inelastic collisions and kinetic-energy bookkeeping.

Where else this chapter is tested

CBSE board isn't the only one — other exams test this chapter too.

RBSE Class 9 Annual (BSER Ajmer)Very high — laws + a momentum numerical almost every year

NTSE / NMMSHigh — Newton's-law MCQs

JEE FoundationVery high — base for Class 11 Laws of Motion

Science Olympiad (NSO)High — force and momentum reasoning

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Questions students ask

The real ones — pulled from the Q&A community and tutor sessions.

Yes in substance. Class 9 (2026-27) uses the new NCF NCERT 'Curiosity' Science book; 'How Forces Affect Motion' covers force, friction, the three laws and momentum. BSER Ajmer sets the RBSE paper.

Because they act on DIFFERENT objects. The reaction on the swimmer (from the water) is what moves the swimmer; the equal action acts on the water, not the swimmer.

Inertia is the property of resisting a change in the state of motion; mass is its quantitative measure. A larger mass has larger inertia.

Because the first and third laws can be derived from it. F = ma (rate of change of momentum) contains the conditions for both rest/uniform motion (F = 0) and interactions.

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