Class 9 Science Chapter 5 Measurement of Length and Motion: 30 MCQs with Answers & Explanations

The 2024–25 CBSE Class 9 Science curriculum prioritizes conceptual clarity over rote learning, and MCQs are the perfect vehicle for this. Unlike descriptive questions, MCQs force you to eliminate incorrect logic and defend your choice in seconds—exactly what Board examiners assess. In Chapter 5, one MCQ might test your grasp of SI units *and* your ability to convert between them. Another might blend rectilinear motion with speed calculations. The new pattern rewards students who can distinguish between definitions (e.g., speed vs. velocity) and apply them instantly. CBSE has shifted away from 'write a paragraph' towards 'identify the correct statement.' This shift rewards precision and conceptual maturity. Moreover, MCQs take 45–60 seconds per question, teaching you time-discipline—essential for the 3-hour Board exam. By mastering MCQs in this chapter, you develop the mental agility needed for Chapters 6–8 (forces, Newton's laws) where one misunderstood concept collapses the entire section.

**Q1.** Which of the following is the SI unit of length? (a) Centimetre (cm) (b) Metre (m) (c) Kilometre (km) (d) Millimetre (mm) **Answer:** (b) Metre (m) **Reason:** The SI base unit for length is metre; all others are derived or non-SI units. **Trap:** Students confuse cm (CGS) or km (practical) with SI. **Q2.** A standard measuring scale is used to measure the length of a pencil. The length is 12 cm. What is this in metres? (a) 0.012 m (b) 0.12 m (c) 1.2 m (d) 120 m **Answer:** (b) 0.12 m **Reason:** 1 cm = 0.01 m, so 12 cm = 0.12 m. **Trap:** Multiplying instead of dividing (mistake: 12 × 0.01 vs. 12 ÷ 100). **Q3.** Which of the following is a standard unit for mass in the SI system? (a) Gram (g) (b) Kilogram (kg) (c) Milligram (mg) (d) Tonne **Answer:** (b) Kilogram (kg) **Reason:** The SI base unit of mass is kilogram; gram is the CGS unit. **Trap:** Many wrongly choose gram because it's commonly used. **Q4.** The SI unit of time is: (a) Hour (h) (b) Minute (min) (c) Second (s) (d) Day (d) **Answer:** (c) Second (s) **Reason:** Second is the internationally agreed SI base unit for time. **Trap:** Hour and minute are practical but not SI units. **Q5.** A car travels 100 metres in 5 seconds. What is its speed? (a) 20 m/s (b) 0.05 m/s (c) 500 m/s (d) 5 m/s **Answer:** (a) 20 m/s **Reason:** Speed = distance ÷ time = 100 ÷ 5 = 20 m/s. **Trap:** Multiplying distance and time (100 × 5 = 500) instead of dividing. **Q6.** Which type of motion is exhibited by a stone falling vertically downward? (a) Circular motion (b) Rectilinear motion (c) Periodic motion (d) Oscillatory motion **Answer:** (b) Rectilinear motion **Reason:** Motion along a straight line in one direction is rectilinear. **Trap:** Students confuse falling motion with oscillatory motion. **Q7.** A ball on a string moving in a circle exhibits: (a) Rectilinear motion (b) Circular motion (c) Periodic motion (d) Both (b) and (c) **Answer:** (d) Both (b) and (c) **Reason:** Circular motion is the path; if it repeats, it's also periodic. **Trap:** Thinking circular and periodic are mutually exclusive. **Q8.** The motion of a swing is an example of: (a) Rectilinear motion (b) Circular motion (c) Periodic motion (d) Random motion **Answer:** (c) Periodic motion **Reason:** A swing repeats its path at regular intervals; this is periodic. **Trap:** Confusing periodic with circular (a swing moves in an arc, but returns to the same position repeatedly). **Q9.** Standard Measuring Instruments measure length most accurately with: (a) Vernier callipers only (b) Ruler and Vernier callipers (c) Ruler only (d) Measuring tape only **Answer:** (b) Ruler and Vernier callipers **Reason:** Ruler gives quick measurements; Vernier callipers give precise sub-millimetre measurements. **Trap:** Believing one instrument is enough for all precision levels. **Q10.** If a boy walks 2 km in 30 minutes, his speed in m/s is: (a) 66.67 m/s (b) 1.11 m/s (c) 3.33 m/s (d) 0.033 m/s **Answer:** (b) 1.11 m/s **Reason:** 2 km = 2000 m; 30 min = 1800 s; Speed = 2000 ÷ 1800 ≈ 1.11 m/s. **Trap:** Forgetting to convert km to m and minutes to seconds.

**Q11.** The SI unit of speed is m/s. Which of the following is equivalent to 1 m/s? (a) 3.6 km/h (b) 0.36 km/h (c) 36 km/h (d) 0.036 km/h **Answer:** (a) 3.6 km/h **Reason:** 1 m/s = (1 × 3600) ÷ 1000 km/h = 3.6 km/h (1 hour = 3600 s, 1 km = 1000 m). **Trap:** Confusing the conversion ratio; many use 10 or 100 incorrectly. **Q12.** A body at rest suddenly starts moving. What is its initial velocity? (a) 0 m/s (b) Undefined (c) 1 m/s (d) Depends on the observer **Answer:** (a) 0 m/s **Reason:** At rest means velocity = 0 by definition; when it starts moving, velocity changes from 0. **Trap:** Thinking initial velocity is undefined or variable. **Q13.** An athlete runs around a 400 m circular track and returns to the starting point in 80 seconds. What is the athlete's speed? (a) 5 m/s (b) 0 m/s (c) 80 m/s (d) 400 m/s **Answer:** (a) 5 m/s **Reason:** Speed = total distance ÷ total time = 400 ÷ 80 = 5 m/s (displacement is zero, but distance is 400 m). **Trap:** Confusing displacement (zero) with distance (400 m); speed uses distance, not displacement. **Q14.** Which statement about rectilinear motion is correct? (a) Object always moves in a straight line at constant speed (b) Object moves in a straight line but speed can change (c) Object moves only horizontally (d) Object must move with uniform velocity **Answer:** (b) Object moves in a straight line but speed can change **Reason:** Rectilinear means straight-line motion; the speed can vary (e.g., accelerating car on a straight road). **Trap:** Wrongly assuming rectilinear motion is always at constant speed. **Q15.** A pendulum completes 20 oscillations in 4 seconds. What is its time period? (a) 0.2 s (b) 0.02 s (c) 5 s (d) 80 s **Answer:** (a) 0.2 s **Reason:** Time period = total time ÷ number of oscillations = 4 ÷ 20 = 0.2 s. **Trap:** Dividing incorrectly (20 ÷ 4 = 5) or forgetting the definition of time period. **Q16.** Which of the following correctly describes periodic motion? (a) Motion that occurs only in circles (b) Motion that repeats itself after regular intervals (c) Motion that is always accelerated (d) Motion that occurs only vertically **Answer:** (b) Motion that repeats itself after regular intervals **Reason:** Periodic motion has a fixed time period; examples include pendulums, heartbeats, seasons. **Trap:** Limiting periodic motion to circular or oscillatory paths only. **Q17.** An object travels 60 m in the first 10 s and 80 m in the next 10 s. What is its average speed? (a) 6 m/s (b) 7 m/s (c) 8 m/s (d) 14 m/s **Answer:** (b) 7 m/s **Reason:** Total distance = 60 + 80 = 140 m; Total time = 10 + 10 = 20 s; Average speed = 140 ÷ 20 = 7 m/s. **Trap:** Averaging the speeds (6 + 8 = 14, then ÷ 2) instead of using total distance ÷ total time. **Q18.** A Vernier callipers has a least count of 0.01 cm. This means: (a) It can measure objects up to 0.01 cm in length (b) Its smallest reading is 0.01 cm (c) It cannot measure below 1 cm (d) It measures only in centimetres **Answer:** (b) Its smallest reading is 0.01 cm **Reason:** Least count (or precision) is the smallest division the instrument can measure accurately. **Trap:** Confusing least count with range or thinking it limits the maximum measurable length. **Q19.** Which of the following is an example of circular motion? (a) A train moving on a straight railway track (b) A planet orbiting the Sun (c) A ball thrown vertically upward (d) A pendulum swinging **Answer:** (b) A planet orbiting the Sun **Reason:** Circular motion occurs when an object moves along a circular path around a fixed centre. **Trap:** Including oscillatory or rectilinear examples as circular motion. **Q20.** If the speed of an object doubles, how does the time taken to cover the same distance change? (a) Time remains the same (b) Time doubles (c) Time halves (d) Time becomes zero **Answer:** (c) Time halves **Reason:** From Speed = distance ÷ time, if speed doubles and distance is constant, time = distance ÷ (2 × speed) = half the original time. **Trap:** Thinking speed and time are directly proportional (they are inversely proportional for a fixed distance).

**Q21. Assertion–Reason:** **Assertion (A):** The SI unit of mass is kilogram, but we often use grams in the laboratory. **Reason (R):** Both kilogram and gram are standard units; their choice depends on the object's mass range. (a) A is true, R is true, R is the correct explanation (b) A is true, R is true, R is NOT the correct explanation (c) A is true, R is false (d) A is false, R is true **Answer:** (b) A is true, R is true, R is NOT the correct explanation **Reason:** Assertion is correct (SI is kg, but g is practical for lab). Reason is partially true but doesn't explain *why* we use grams—the real reason is practicality (kg is too large for small lab samples). **Trap:** Confusing practical convenience with the definition of SI units. **Q22. Assertion–Reason:** **Assertion (A):** A body moving in a circle at constant speed is undergoing rectilinear motion. **Reason (R):** Rectilinear motion means motion in a straight line. (a) A is true, R is true, R is the correct explanation (b) A is true, R is true, R is NOT the correct explanation (c) A is false, R is true (d) A is false, R is false **Answer:** (c) A is false, R is true **Reason:** Circular motion is NOT rectilinear; the reason is correct, so A is false and R is true. **Trap:** Thinking constant speed = rectilinear motion (speed alone doesn't determine the path type). **Q23.** A car travels from City A to City B (100 km apart) in 2 hours. It returns from B to A in 1.5 hours via a different 80 km route. What is the average speed for the entire journey? (a) 51.4 m/s (b) 51.4 km/h (c) 100 km/h (d) 90 km/h **Answer:** (b) 51.4 km/h **Reason:** Total distance = 100 + 80 = 180 km; Total time = 2 + 1.5 = 3.5 hours; Average speed = 180 ÷ 3.5 ≈ 51.4 km/h. **Trap:** Averaging the two speeds (50 + 53.3 ÷ 2) or ignoring the different distances and times. **Q24. Assertion–Reason:** **Assertion (A):** Displacement of a body can be zero even if the distance travelled is not zero. **Reason (R):** Displacement is the shortest path between initial and final positions; distance is the total path length. (a) A is true, R is true, R is the correct explanation (b) A is true, R is true, R is NOT the correct explanation (c) A is false, R is true (d) Both A and R are false **Answer:** (a) A is true, R is true, R is the correct explanation **Reason:** A body returns to its starting point (e.g., running around a track); displacement = 0 but distance > 0; R correctly defines both. **Trap:** Confusing displacement with distance or thinking they are always equal. **Q25.** A pendulum has a frequency of 2 Hz. How many oscillations will it complete in 1 minute? (a) 2 (b) 30 (c) 60 (d) 120 **Answer:** (d) 120 **Reason:** Frequency = oscillations ÷ time; 2 Hz means 2 oscillations per second; in 60 seconds, 2 × 60 = 120 oscillations. **Trap:** Confusing frequency (oscillations per second) with time period (time per oscillation); many use 1 ÷ 2 = 0.5 as time period, then multiply by 60. **Q26.** An object starts from rest and travels 20 m in the first 2 seconds, then 60 m in the next 3 seconds. Which statement is correct? (a) The object is moving with uniform speed throughout (b) The object is accelerating (c) The speed in first 2 s is 10 m/s; in next 3 s is 20 m/s (d) Both (b) and (c) **Answer:** (d) Both (b) and (c) **Reason:** Speed₁ = 20 ÷ 2 = 10 m/s; Speed₂ = 60 ÷ 3 = 20 m/s; speeds differ, so the object is accelerating. **Trap:** Assuming constant speed because distance changes proportionally; actually, changing speed indicates acceleration. **Q27. Assertion–Reason:** **Assertion (A):** The time period of a simple pendulum is independent of the mass of the bob. **Reason (R):** The SI unit of time is second, which does not depend on mass units. (a) A is true, R is true, R is the correct explanation (b) A is true, R is true, R is NOT the correct explanation (c) A is true, R is false (d) A is false, R is true **Answer:** (c) A is true, R is false **Reason:** Assertion is experimentally true (T = 2π√(L/g), independent of mass). Reason is illogical (SI units don't explain physical independence from mass). **Trap:** Accepting a plausible-sounding reason even if it doesn't logically support the assertion. **Q28.** A runner completes a 400 m lap around a track in 50 seconds, maintains uniform speed, and continues for 5 laps. What is the total distance after 3 full laps? (a) 400 m (b) 800 m (c) 1200 m (d) 2000 m **Answer:** (c) 1200 m **Reason:** Distance = 3 laps × 400 m/lap = 1200 m. **Trap:** Confusing laps with time; multiplying by time (50 s) instead of metres per lap. **Q29.** Which of the following instruments would give the most accurate measurement of the diameter of a thin wire? (a) Metre scale (b) Measuring tape (c) Vernier callipers (d) Screw gauge (micrometer) **Answer:** (d) Screw gauge (micrometer) **Reason:** Screw gauge has the smallest least count (0.01 mm), making it ideal for measuring thin wire diameters. **Trap:** Choosing Vernier callipers (0.01 cm or 0.1 mm) instead of screw gauge, which is more precise. **Q30.** A satellite orbits Earth at a constant altitude. If it completes one orbit in 24 hours, its motion is: (a) Rectilinear and periodic (b) Circular but not periodic (c) Periodic but not circular (d) Both circular and periodic **Answer:** (d) Both circular and periodic **Reason:** The path is circular (around Earth); the motion repeats every 24 hours (period = 24 h), so it's both circular and periodic. **Trap:** Thinking circular and periodic are mutually exclusive or that circular motion can't be periodic.

**Trap 1: Confusing SI Units with Practical Units** Many students choose gram instead of kilogram, or cm instead of metre, because these are commonly used in daily life. Remember: CBSE exams test *standard SI units*. Gram is CGS (centimetre–gram–second), not SI. Always cross-check: Is this the *international standard* or just locally convenient? **Trap 2: Speed ÷ Time Instead of Distance ÷ Time** Students sometimes reverse the speed formula. Keep this mantra: "Speed eats distance and time for breakfast." Speed = distance ÷ time. Never divide distance by speed or multiply distance and time. **Trap 3: Confusing Displacement with Distance** Displacement is a vector (has direction and is the straight-line shortest path); distance is a scalar (always positive, total path length). A runner who completes a lap returns to the starting point: displacement = 0, distance = lap length. This distinction underlies many Board questions. **Trap 4: Assuming All Circular Motion is Periodic** Circular motion *can be* periodic (e.g., planets, satellites), but it's not always. A sling swinging in a circle at varying speed is circular but not periodic. Always check: Does the motion repeat at regular intervals? **Trap 5: Misidentifying Motion Types** Rectilinear = straight line (not just horizontal; can be vertical). Circular = around a fixed centre. Periodic = repeats at regular time intervals (e.g., heartbeats, clock pendulum). A falling object is rectilinear (vertical straight line), not periodic or circular. **Trap 6: Averaging Speeds Incorrectly** For average speed, use: Average speed = **total distance ÷ total time**, NOT the arithmetic mean of individual speeds. If a car travels 100 m in 10 s (speed = 10 m/s) then 100 m in 20 s (speed = 5 m/s), average speed ≠ (10 + 5) ÷ 2 = 7.5 m/s. Correct: (100 + 100) ÷ (10 + 20) = 200 ÷ 30 ≈ 6.67 m/s. **Trap 7: Ignoring Unit Conversions** If a question gives distance in km and time in minutes, convert *before* calculating. 2 km = 2000 m; 30 minutes = 1800 seconds. Many students forget this and get wildly incorrect answers (e.g., speed = 2 ÷ 30 ≈ 0.067 instead of the correct 1.11 m/s). **Trap 8: Confusing Time Period with Frequency** Time period (T) is time for one oscillation; frequency (f) is number of oscillations per second. They are inverses: f = 1 ÷ T and T = 1 ÷ f. If frequency = 2 Hz, time period = 0.5 s (not 2 seconds).

The CBSE Class 9 Science exam allocates ~90 minutes for 30 questions (3 marks per question typically, but MCQs may carry 1 mark each). Here's a proven strategy: **Read-Analyze-Decide Framework (45–60 seconds per question):** 1. **Read once, identify keywords** (15 seconds): Circle SI unit, distance, circular, periodic, speed formula. 2. **Eliminate two obviously wrong options** (15 seconds): In 'SI unit of mass,' you can instantly rule out gram and tonne. 3. **Choose between remaining two** (15 seconds): Compare both against the question. If unsure, re-read the question stem—the answer is always there. 4. **Move forward** (0 seconds regret): Don't second-guess. Confidence and speed beat perfectionism in MCQs. **Question-Order Strategy:** - **First pass (Easy):** Solve Q1–Q10 (Foundational) in ~8 minutes. These build confidence and secure 10/10 marks. - **Second pass (Medium):** Tackle Q11–Q20 (Conceptual) in ~12 minutes. These require thought but are high-yield. - **Third pass (Hard):** Save Q21–Q30 (Assertion–Reason, synthesis) for last. Allocate ~15 minutes; if you finish, review. - **Buffer:** Reserve 5 minutes to revisit any flagged questions. **During the Exam:** - **Flag, don't linger:** If unsure after 45 seconds, mark your best guess and flag it. Return only if time permits. - **Trust the format:** CBSE designs one correct answer per question. If two seem correct, re-read the question—one will contradict or be incomplete. - **Use answer choices strategically:** For conversion questions (Q10), work backward: multiply the given distance by each option's time conversion and see which yields a sensible speed. - **For Assertion–Reason (Q21, Q22, Q24, Q27):** Read R *first*. If R is clearly false, eliminate (c) and (d). Then check if A is correct. This two-step approach is faster. **Common Exam Pitfalls:** - Spending 2 minutes on one hard MCQ while easy ones remain unsolved. - Recalculating answers three times (wastes 10+ minutes). - Panicking over unfamiliar wording (all MCQs use NCERT vocabulary; stay calm). **Practice Protocol for Mastery:** Take three full 30-question timed mock tests (one per week) before your half-yearly. Track your time-per-question and error patterns. If you consistently misread 'speed' as 'displacement,' highlight that word in future questions.

This resource mirrors the CBSE 2024–25 Science Chapter 5 syllabus exactly. Here's how to integrate it into your study plan: **Week 1 (Learning):** Read NCERT Chapter 5 once. Then solve Easy MCQs (Q1–Q10) to confirm foundational understanding. Aim for 10/10. **Week 2 (Consolidation):** Solve Medium MCQs (Q11–Q20) without a timer first. Review explanations. Then retake with a timer to build speed. **Week 3 (Mastery):** Solve all 30 MCQs in one sitting (90 minutes). Score yourself. If <27/30, identify weak topics and re-read relevant NCERT sections. If ≥27/30, you're Board-ready. **Daily 10-Minute Drill:** Pick 3–5 random MCQs, solve in 3 minutes, review in 2 minutes. This maintains sharpness week-to-week. **Before Half-Yearly Exam:** Solve Easy tier once more (confidence booster), then one full 30-question timed mock. Every MCQ here aligns with the NCERT textbook and CBSE mark scheme. Common misconceptions are embedded in trap options—recognizing these *now* prevents exam-day blunders. Your goal is not to memorize answers but to understand *why* one option is correct and why three are not. That depth is what separates 70% scorers from 95%+ scorers in Board exams.