Class 9 Science Chapter 9 Friction Previous Year Questions – Solved PYQs 2020–2025

Reading Chapter 9 once more won't improve your exam score—solving past papers will. Here's why: (1) **Recall under pressure**: Exams test memory and speed, not understanding alone. When you solve a 1-mark question in 30 seconds, you're training your brain to retrieve 'static friction > kinetic friction' instantly, not thoughtfully. (2) **Recognizing question patterns**: CBSE examiners repeat certain framings. For example, 'Name the type of friction acting when a book slides on a table' appears almost every year in different wording. Past papers reveal these patterns. (3) **Calibrating difficulty**: Chapter 9 has both trivial and deceptive questions. A 3-mark question might ask *why* oil reduces friction (causes: → molecular-level separation, reduced adhesion, lower contact pressure), not just *that* it does. Only past papers show this gradation. (4) **Time management**: You get 3 hours to answer 40 marks of Science. Solving timed PYQs teaches you whether to spend 2 minutes or 4 on a friction question. (5) **Spotting your gaps**: After solving 15 friction PYQs, you'll know if you muddle static and kinetic friction, or misunderstand fluid friction in viscosity contexts. Theory reading doesn't expose these gaps as quickly.

**Question 1: Define static friction. Is it a fixed force?** Answer: Static friction is the friction that acts on a body at rest relative to a surface, preventing it from moving when an external force is applied. It is *not* a fixed force—it increases from zero up to a maximum value (μₛN) as the applied force increases, until the body just begins to move. Once applied force exceeds maximum static friction, the body starts sliding. **Question 2: A wooden block is pushed gently along a horizontal table. Name the friction acting and its direction.** Answer: The friction acting is kinetic (or sliding) friction. Its direction is *opposite* to the motion of the block (or opposite to the applied force). **Question 3: Distinguish between static and kinetic friction in one sentence.** Answer: Static friction acts on a stationary object and can vary up to a maximum; kinetic friction acts on a moving object and is approximately constant. **Question 4: Why do athletes wear spiked shoes on a running track?** Answer: Spikes increase the effective contact area (or roughness) between the shoe and track, increasing the coefficient of friction, which increases the grip and prevents slipping during acceleration and direction changes. **Question 5: What type of friction acts on a ball rolling on grass?** Answer: Rolling friction acts on the ball. Rolling friction is much smaller than sliding friction because the contact area is minimal and no relative sliding occurs.

**Question 1: Explain with an example how friction can be both useful and harmful.** Answer: *Useful*: Friction between shoe soles and ground allows us to walk without slipping; between tyre and road enables a car to move and brake. *Harmful*: Friction in machine parts (gears, bearings) causes wear and energy loss, requiring regular lubrication. In both cases, the *magnitude* of friction and its direction determine benefit or harm. (Marks: 1 for useful example, 1 for harmful example, 1 for explanation of why both occur.) **Question 2: A book is placed on an inclined plane. As the angle of inclination is increased slowly, the book first remains stationary, then starts to slide. Explain why.** Answer: When the plane is nearly horizontal, the component of weight down the slope is small, and static friction (which can be up to μₛN) balances it. As the angle increases, this component increases. When the weight component down the slope exceeds the maximum static friction (μₛN), static friction cannot hold the book anymore, and it begins to slide. Once sliding, kinetic friction (smaller than maximum static friction) acts, and the book accelerates downward. **Question 3: Define the coefficient of friction. Is it a constant for a given pair of surfaces?** Answer: The coefficient of friction (μ) is the ratio of the friction force to the normal force: μ = f/N. For a given pair of surfaces, the coefficient is *approximately* constant but depends on the texture, temperature, and nature of both surfaces. It does not depend on the area of contact or the normal force separately. **Question 4: Why is a motor oil used to lubricate machinery? Give two reasons.** Answer: (1) Oil fills the gaps between rough surfaces, creating a thin film that separates the surfaces, reducing direct contact and thus reducing adhesion and friction. (2) Oil allows the surfaces to slip over each other more smoothly, reducing the coefficient of friction and the energy dissipated as heat. (3) Oil carries away heat generated by friction, preventing overheating. **Question 5: A book is on a table. The maximum static friction is 5 N. If you pull the book with 3 N horizontally, will it move? If you pull with 6 N, what happens?** Answer: At 3 N: No, the book will not move because the applied force (3 N) is less than the maximum static friction (5 N). Static friction will equal 3 N and balance the pull. At 6 N: Yes, the book will start moving because the applied force (6 N) exceeds maximum static friction. Once moving, kinetic friction (< 5 N) acts, and the book accelerates in the direction of the 6 N pull.

**Question 1: A boy drags a wooden box across a rough floor by a rope inclined at 30° to the horizontal. The mass of the box is 20 kg. The coefficient of kinetic friction is 0.25. Calculate the tension in the rope if the box moves at constant velocity. (g = 10 m/s²)** *Full Solution*: Since the box moves at constant velocity, net force = 0 (equilibrium). Forces acting: - Weight (W) = mg = 20 × 10 = 200 N (downward) - Normal reaction (N) (upward) - Tension (T) at 30° to horizontal → components: T cos(30°) horizontal, T sin(30°) vertical - Kinetic friction (f) = μₖN (opposing motion, horizontal) Vertical equilibrium: N + T sin(30°) = mg N + T × 0.5 = 200 N = 200 − 0.5T … (1) Friction: f = μₖN = 0.25 × (200 − 0.5T) = 50 − 0.125T … (2) Horizontal equilibrium: T cos(30°) = f T × (√3/2) = 50 − 0.125T T × 0.866 = 50 − 0.125T T × 0.866 + T × 0.125 = 50 T × 0.991 = 50 T ≈ 50.5 N **Answer: Tension ≈ 50.5 N** --- **Question 2: Explain the phenomenon of 'stick-slip' motion with reference to static and kinetic friction. Why does a squeaking door hinge exhibit this?** *Full Solution*: *Stick-slip motion* occurs when a body alternates between stationary (sticking) and sliding (slipping) phases. Explanation: When you try to drag an object, the applied force first increases static friction until it reaches its maximum (μₛN). If applied force exceeds this, the object suddenly jerks into motion. Once moving, kinetic friction (μₖN) takes over, which is usually *less* than maximum static friction. This sudden decrease causes the body to accelerate briefly. If the pulling speed is then reduced or if surface irregularities intervene, the applied force may fall below kinetic friction, and the body halts again—static friction re-engages, and the cycle repeats. In a squeaking door hinge: As you open the door, kinetic friction is overcome → the hinge turns smoothly. If you slow down or the force drops, static friction suddenly re-grips the hinge pin → the door momentarily stops. As you continue pushing, the force exceeds static friction again → the hinge lurches forward. These repeated stick-slip cycles cause vibrations that produce the squeak sound at the natural resonance frequency of the hinge. Solution: Apply oil or grease to reduce both μₛ and μₖ, and narrow their difference, smoothing the transition. --- **Question 3: A sphere rolls without slipping down an inclined plane of angle θ. Explain why rolling friction is less than sliding friction. How does this relate to the normal force?** *Full Solution*: *Rolling friction vs. sliding friction*: When a sphere *slides* down (without rolling), kinetic friction acts over the entire contact area. The friction force f = μₖN acts tangentially opposite to motion, dissipating energy. When the sphere *rolls without slipping*, the contact point has *zero* instantaneous velocity (no relative motion at that point). Rolling friction arises not from sliding but from the deformation of the sphere and plane at the contact point. This deformation creates a small "dent" that the sphere must roll over, but the friction force acts over a negligible distance per unit of motion. Consequently, rolling friction f_roll = μ_r N is much smaller than kinetic friction (μ_r << μₖ). *Relationship to normal force*: Both rolling and sliding friction are *proportional* to the normal force N. However: - Sliding friction: f = μₖN (high energy dissipation) - Rolling friction: f_roll = μ_r N (low energy dissipation) On an incline, the normal force N = mg cos(θ). If you double the mass, both frictions double, but rolling friction remains much smaller. This is why spheres roll with little energy loss—the frictional force is small even though N is substantial. Example: A steel ball rolling on a hard floor travels farther than a wooden block sliding on the same floor, even though both experience friction proportional to N. **Answer: Rolling friction is minimal because contact is instantaneously stationary (no sliding), reducing energy dissipation; both scale with N, but the coefficient for rolling (μ_r) is far smaller than for sliding (μₖ).**

Based on recent CBSE advisory notes and pilot question banks, the 2026–27 Science paper is expected to emphasize *application* and *numerics* over rote definitions. For Chapter 9, watch for these shifts: **1. Increased focus on fluid friction and viscosity**: The older syllabus treated fluid friction as an afterthought. The revised 2024–25 NCERT gives it more weight. Expect questions on terminal velocity, viscosity of liquids, and why parachutes slow down. One 3-mark or 5-mark question on this is now likely. **2. Real-world engineering contexts**: Instead of abstract 'boxes on tables,' questions may ask: 'Why do ball bearings reduce friction in skateboard wheels?' or 'How does brake fluid viscosity affect braking distance?' These test conceptual depth, not memorization. **3. Numerical problems with multiple steps**: A 5-mark friction question will increasingly require you to (a) draw a free-body diagram, (b) resolve forces, (c) apply f = μN, and (d) calculate or compare. Single-concept numerics are becoming rare. **4. Assertion-Reason format (rare but emerging)**: A few schools report seeing assertion-reason friction questions in mock papers. Example: 'Assertion: Kinetic friction is always less than maximum static friction. Reason: Once an object moves, the coefficient of friction decreases.' (Both true; Reason explains Assertion.) **5. 'Explain with a diagram' questions**: The 2026–27 pattern is pushing students to sketch free-body diagrams, inclined planes, or rolling scenarios. A 3-mark question now often rewards a clear diagram (1 mark) + explanation (2 marks). **Action for students**: Beyond solving old PYQs, practice numerics, draw diagrams, and revisit fluid friction examples in your NCERT. Mock papers from schools following the new pattern will be invaluable.

**Time allocation**: Friction is typically 4–6 marks of your 40-mark Science paper (10–15% weight). Budget 8–12 minutes total: roughly 1 minute per 1-mark, 2–3 minutes per 3-mark, and 4–5 minutes per 5-mark. **For 1-mark questions**: - Read carefully: 'Define,' 'Name,' 'State,' or 'Give reason' require different answer formats. - 'Name the type of friction' → One word or phrase: 'sliding,' 'static,' 'rolling,' 'fluid.' Do not explain unless asked. - If unsure, use context: A *sliding* object has kinetic friction; a *stationary* object has static friction; a *rolling* object has rolling friction. **For 3-mark questions**: - Spend 10–15 seconds reading and planning. Identify if it's 'explain,' 'distinguish,' 'calculate,' or 'apply.' - 'Explain' → Give reason with example. 'Distinguish' → Two-column or side-by-side comparison. 'Calculate' → Show formula, substitute, and answer. - Always include *one concrete example*. Examiners reward specificity: 'walking on a floor' is better than 'movement.' - Use bullet points or short sentences; examiners value clarity, not flowery prose. **For 5-mark questions**: - First, draw a diagram (free-body, incline, or scenario). This earns 1 mark and anchors your solution. - List given data and unknowns. This prevents silly errors. - Apply step-by-step logic. Friction questions usually require: (1) identify forces, (2) apply Newton's laws or equilibrium, (3) calculate. - For *stick-slip* or *conceptual* 5-mark questions, structure as: (1) Define the phenomenon, (2) Explain the physics (use μₛ vs. μₖ), (3) Give a real-world example, (4) Mention a solution or implication. **Common pitfalls to avoid**: - Confusing direction: Friction always opposes motion (or tendency). If motion is to the right, friction is to the left. - Forgetting normal force: f = μN. If an object is on an incline, N ≠ mg unless the plane is horizontal. - Treating μ as variable: Coefficient is constant for a pair of surfaces; friction force *is* variable (depends on N and whether object is moving). - Ignoring the question demand: A question asking for 'one reason' rewards one reason. Giving three may lose marks due to verbosity. **Final 2-minute check**: - Units correct? (Force in N, acceleration in m/s², etc.) - Sign correct? (Friction opposes motion.) - Formula applied correctly? (f = μN, not f = μ/N.) - Answer reasonable? (Friction on a book shouldn't be 1000 N; static friction > kinetic for the same surfaces.) Start a 3-day free trial at cbsetutor.ai to practise these strategies with personalized feedback and timed mock tests on Chapter 9.

**Friction formulas**: - Kinetic friction: f_k = μₖ × N - Maximum static friction: f_s,max = μₛ × N - Normal force on horizontal surface: N = mg - Normal force on incline at angle θ: N = mg cos(θ) - Component of weight down incline: mg sin(θ) **Key relationships**: - Coefficient of static friction (μₛ) > coefficient of kinetic friction (μₖ) for most surfaces. - Friction does *not* depend on contact area (for a given normal force). - Friction is roughly independent of speed (once sliding begins). - Rolling friction coefficient (μ_r) << kinetic friction coefficient (μₖ). **Types of friction** (in order of typical magnitude): 1. Fluid friction (air, water) — smallest for high speed; can dominate at low speeds (viscous drag). 2. Rolling friction — very small; used in ball bearings, wheels. 3. Kinetic (sliding) friction — moderate; acts when surfaces slide past each other. 4. Static friction — can range from zero up to maximum; prevents motion initially. **Terminal velocity**: When an object falls through a fluid, air resistance increases with speed. At terminal velocity, air resistance = weight, so net force = 0 and acceleration = 0. Friction (air resistance) is no longer a 'nuisance' but a balancing force. **Methods to reduce friction**: - Lubrication (oil, grease) → reduces μ by separating surfaces. - Polishing surfaces → reduces roughness → reduces μ. - Ball bearings / rollers → converts sliding to rolling, reducing f. - Streamlining → reduces drag in fluids. **Methods to increase friction**: - Roughen surfaces (sandpaper, spikes) → increases μ. - Increase normal force (press down harder). - Use high-μ materials (rubber vs. ice).