Class 9 Physics Chapter 4 Laws of Motion: 13 Solved Previous Year Questions (2020–2025)

NCERT Chapter 4 introduces Newton's First Law (inertia and force), Second Law (F=ma and momentum), and Third Law (action-reaction pairs). You'll also study friction, types of friction, and their real-world applications. These concepts form the foundation for Class 10 and Class 11 Physics. Understanding these laws is essential because they explain everyday phenomena—why you lurch forward when a bus brakes, why a batsman needs to complete the swing, and why rockets propel forward. Mastery of these topics directly impacts your board exam performance and competitive exam readiness.

Our curated set includes 4 easy-level questions (1–2 marks) focusing on concept definitions and direct formula application, 5 medium-level questions (3–4 marks) involving calculations and multi-step reasoning, and 4 advanced-level questions (5 marks) combining multiple laws and real-world scenarios. This three-tier structure ensures you build confidence gradually. Each solution follows CBSE marking scheme guidelines, showing all working and reasoning. Questions cover Newton's laws, friction (static and kinetic), motion on inclined planes, connected bodies, and momentum conservation—the full spectrum of Chapter 4.

Questions on Newton's First Law typically ask you to explain inertia, define force, or describe everyday examples. A common 2020–2022 question pattern: 'Why does a person fall forward when a moving bus suddenly stops?' The answer hinges on understanding that the bus exerts a backward force, but your body, due to inertia, continues forward. NCERT emphasizes the mathematical definition: F = rate of change of momentum. These problems rarely require calculations but demand conceptual clarity. Studying solved examples helps you articulate why objects persist in their state of motion without external force—critical for both exams and physics intuition.

F = ma is the heartbeat of Chapter 4. Previous year questions test your ability to find acceleration, net force, or mass in multi-step scenarios. For example: 'A 2 kg block is pushed with a force of 20 N. If friction is 4 N, find acceleration.' Here, net force = 20 – 4 = 16 N, so a = 16/2 = 8 m/s². More complex questions involve two or three objects, requiring you to draw free-body diagrams and apply F = ma to each separately. NCERT stresses that force is a vector; direction matters. Solutions must show your understanding of net force, not just plug-and-chug calculation. Practicing these ensures you score full marks on calculation-based board questions.

The Third Law—'Every action has an equal and opposite reaction'—is often misunderstood. Common misconense: if forces are equal, why doesn't an object stay stationary? The answer: action-reaction pairs act on *different* objects. A solved question might ask: 'A person pushes a wall with force F. What is the reaction, and on what object does it act?' The reaction (force F) acts on the person, exerted by the wall. These conceptual questions appear in 2–3 mark slots. NCERT Chapter 4 emphasizes that action-reaction pairs never cancel because they don't act on the same body. Understanding this distinction is crucial for board success and prevents common exam errors.

Friction questions make up 20–25% of Chapter 4 board exams. You must know: static friction increases up to a limit, kinetic friction is constant once motion starts, and μs > μk (coefficients differ). A typical 4-mark problem: 'A 5 kg block rests on a horizontal surface (μs = 0.4, μk = 0.3, g = 10 m/s²). How much force is needed to start motion? Once moving, if the applied force is 20 N, find acceleration.' Solution requires calculating maximum static friction (20 N), then using F_net = Applied – Kinetic Friction for acceleration. NCERT provides formulas; solved examples teach you when to apply which. Mastering friction prepares you for inclined plane and connected body problems.

Inclined plane questions blend all Chapter 4 concepts: decomposing weight into parallel and perpendicular components, calculating normal force and friction, applying F = ma along the plane. A solved example: 'A 10 kg block slides down a 30° incline. If μk = 0.2, find acceleration.' Solution: mg sin(30°) = 50 N down the plane, normal force N = mg cos(30°) ≈ 85 N, friction f = μk × N ≈ 17 N. Net force = 50 – 17 = 33 N, so a ≈ 3.3 m/s². NCERT Chapter 4 diagrams and exercises build this step-by-step. Board exams often include a 5-mark inclined plane question; practicing solved versions ensures you draw correct diagrams and avoid sign errors.

Problems involving two or more connected objects (via string, pulley, or contact) are among the toughest in Chapter 4. Example: 'Two blocks, A (2 kg) and B (3 kg), are connected by a light string over a pulley. B hangs vertically; A is on a frictionless horizontal surface. Find tension and acceleration.' Solution requires writing F = ma for each block separately, then solving simultaneously. For B: mg – T = ma; for A: T = ma. Combining gives acceleration a = (m_B × g) / (m_A + m_B). These questions test your ability to visualize forces, apply laws correctly to each object, and solve linear equations. NCERT Chapter 4 includes such problems; practicing multiple solved variants builds confidence for 5-mark board questions.

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Success on Chapter 4 questions depends on: (1) Always draw free-body diagrams showing all forces with correct directions. (2) Identify the system and apply F = ma consistently. (3) For multi-object problems, write equations for each object separately before combining. (4) Check units and signs—Newton (N) for force, kg for mass, m/s² for acceleration. (5) Read the question carefully; distinguish between static and kinetic friction. (6) Solve symbolically first, then substitute numbers—this prevents arithmetic errors. NCERT examples demonstrate these practices. Board examiners award marks for methodology, not just final answers; showing your reasoning via diagrams and equations is essential. Practice solved previous year questions using these strategies to maximize your score.

Misconception 1: 'A heavier object falls faster.' Truth: In the absence of air resistance, all objects accelerate at g (9.8 m/s²), regardless of mass—Newton's Second Law explains this. Misconception 2: 'Action and reaction cancel out, so nothing moves.' Truth: They act on *different* objects and therefore don't cancel. Misconception 3: 'Friction always opposes motion.' Truth: Static friction can aid motion (e.g., a car accelerating—friction between tire and road provides the forward force). NCERT Chapter 4 clarifies these points through definitions and examples. Understanding the distinction between these ideas is why students who solve multiple previous year questions often score better—repetition reinforces correct mental models.