Describing Motion Around Us — Class 9 Science NCERT Solutions (Free)
Free step-by-step NCERT solutions for Class 9 Science chapter "Describing Motion Around Us" — 8 important questions with detailed answers for CBSE board exam preparation.
TL;DR: Free step-by-step NCERT solutions for Class 9 Science chapter "Describing Motion Around Us" — 8 important questions with detailed answers for CBSE boa…
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NCERT Solutions for Class 9 Science — Describing Motion Around Us. Step-by-step answers to all 8 textbook questions from this chapter, written for CBSE board preparation and free to use.
Describing Motion Around Us — All 8 Questions Solved
Q1. Define distance and displacement.
Distance is the total path length traveled by an object (scalar quantity, always positive).
Displacement is the straight-line distance between starting and ending positions with direction (vector quantity, can be positive or negative).
Q2. A car travels 5 km east, then 3 km west. Find distance and displacement.
Distance = 5 + 3 = 8 km
Displacement = 5 - 3 = 2 km (east)
Note: Distance is always ≥ magnitude of displacement.
Q3. Define speed and velocity.
Speed = total distance / total time (scalar, always positive)
Velocity = displacement / time with direction (vector)
Formula: v = s/t
Unit: m/s
Q4. A cyclist covers 60 m in 5 seconds. Calculate speed.
Speed = Distance / Time = 60 m / 5 s = 12 m/s
Q5. Define acceleration and write its formula.
Acceleration is the rate of change of velocity with time.
Formula: a = (v - u) / t
Where u = initial velocity, v = final velocity, t = time
Unit: m/s²
Q6. A car accelerates from 0 m/s to 20 m/s in 4 seconds. Find acceleration.
Step 1: u = 0 m/s, v = 20 m/s, t = 4 s
Step 2: a = (v - u) / t = (20 - 0) / 4 = 5 m/s²
Q7. State the three equations of motion.
1. v = u + at
2. s = ut + ½at²
3. v² = u² + 2as
Where u = initial velocity, v = final velocity, a = acceleration, t = time, s = displacement
Q8. A ball is dropped from rest (u = 0). After 3 seconds, find velocity (g = 10 m/s²).
Step 1: u = 0, a = g = 10 m/s², t = 3 s
Step 2: v = u + at = 0 + 10(3) = 30 m/s
Describing Motion Around Us — Practice MCQs with Answers
Attempt these 20 multiple-choice questions after working through the solutions above. Each carries the correct option and the reasoning behind it, so a wrong answer tells you which idea to revisit.
MCQ 1. When is an object said to be in motion?
- A. When its position changes with respect to a stationary object.
- B. When its position remains constant over time.
- C. When it is acted upon by a force.
- D. When its color changes.
Answer: A. When its position changes with respect to a stationary object. — An object is considered to be in motion if it changes its position with respect to its surroundings or a reference point over a period of time.
MCQ 2. Which of the following quantities is a scalar quantity?
- A. Displacement
- B. Velocity
- C. Speed
- D. Acceleration
Answer: C. Speed — Speed is a scalar quantity as it only has magnitude. Displacement, velocity, and acceleration are vector quantities, possessing both magnitude and direction.
MCQ 3. A car travels 5 km North, then 12 km East. What is the total distance covered by the car?
- A. 13 km
- B. 7 km
- C. 17 km
- D. 10 km
Answer: C. 17 km — Total distance covered is the sum of the magnitudes of all individual paths taken. So, 5 km + 12 km = 17 km.
MCQ 4. A person walks 4 m East and then 3 m North. What is the magnitude of the person's displacement?
- A. 7 m
- B. 5 m
- C. 1 m
- D. 12 m
Answer: B. 5 m — Displacement is the shortest distance between the initial and final positions. For paths at right angles, it can be found using the Pythagorean theorem: sqrt(4^2 + 3^2) = sqrt(16 + 9) = sqrt(25) = 5 m.
MCQ 5. What is the unit of speed in the SI system?
- A. m/s^2
- B. km/h
- C. m/s
- D. cm/s
Answer: C. m/s — The SI unit of speed is metres per second (m/s). Kilometres per hour (km/h) is another common unit, but not the SI unit.
MCQ 6. A bus travels the first 100 km of its journey at an average speed of 50 km/h and the next 100 km at an average speed of 25 km/h. What is the average speed of the bus for the entire journey?
- A. 37.5 km/h
- B. 33.33 km/h
- C. 40 km/h
- D. 30 km/h
Answer: B. 33.33 km/h — Time for first 100 km = 100 km / 50 km/h = 2 hours. Time for next 100 km = 100 km / 25 km/h = 4 hours. Total distance = 200 km. Total time = 6 hours. Average speed = 200 km / 6 h = 33.33 km/h.
MCQ 7. A car starts from rest and accelerates uniformly to a velocity of 20 m/s in 5 seconds. It then travels at a constant velocity for 10 seconds and finally decelerates uniformly to rest in another 5 seconds. What is the average velocity of the car during the entire motion?
- A. 10 m/s
- B. 12 m/s
- C. 15 m/s
- D. 13.33 m/s
Answer: C. 15 m/s — Phase 1 displacement: s1 = (0+20)/2 * 5 = 50 m. Phase 2 displacement: s2 = 20 * 10 = 200 m. Phase 3 displacement: s3 = (20+0)/2 * 5 = 50 m. Total displacement = 50 + 200 + 50 = 300 m. Total time = 5 + 10 + 5 = 20 s. Average velocity = 300 m / 20 s = 15 m/s.
MCQ 8. In uniform motion, an object covers:
- A. Equal distances in unequal intervals of time.
- B. Unequal distances in equal intervals of time.
- C. Equal distances in equal intervals of time.
- D. Unequal distances in unequal intervals of time.
Answer: C. Equal distances in equal intervals of time. — An object is said to be in uniform motion if it travels equal distances in equal intervals of time, regardless of how small these time intervals may be.
MCQ 9. What does a negative acceleration signify?
- A. The object is speeding up.
- B. The object is at rest.
- C. The object is slowing down.
- D. The object is moving at constant velocity.
Answer: C. The object is slowing down. — Negative acceleration, also known as retardation or deceleration, means that the velocity of the object is decreasing over time.
MCQ 10. The SI unit of acceleration is:
- A. m/s
- B. km/h^2
- C. m/s^2
- D. N/kg
Answer: C. m/s^2 — Acceleration is the rate of change of velocity. Since velocity is in m/s and time is in s, the unit of acceleration is m/s/s or m/s^2.
MCQ 11. A distance-time graph shows a straight line parallel to the time axis. What does this indicate about the object's motion?
- A. The object is moving with uniform velocity.
- B. The object is at rest.
- C. The object is moving with uniform acceleration.
- D. The object is moving with non-uniform velocity.
Answer: B. The object is at rest. — A straight line parallel to the time axis on a distance-time graph means that the distance covered by the object is not changing over time, implying the object is stationary or at rest.
MCQ 12. A velocity-time graph shows a straight line parallel to the time axis. What does this indicate about the object's motion?
- A. The object is at rest.
- B. The object is moving with uniform acceleration.
- C. The object is moving with uniform velocity.
- D. The object is moving with non-uniform acceleration.
Answer: C. The object is moving with uniform velocity. — A straight line parallel to the time axis on a velocity-time graph indicates that the velocity of the object is constant over time. Therefore, the object is moving with uniform velocity and zero acceleration.
MCQ 13. A stone is thrown vertically upward with an initial velocity of 20 m/s. If the acceleration due to gravity is 10 m/s^2 downwards, what will be its velocity after 1 second?
- A. 10 m/s upwards
- B. 30 m/s upwards
- C. 10 m/s downwards
- D. 0 m/s
Answer: A. 10 m/s upwards — Using v = u + at, where u = 20 m/s, a = -10 m/s^2 (downwards is negative), t = 1 s. v = 20 + (-10)(1) = 10 m/s. The positive sign indicates velocity is still upwards.
MCQ 14. A car starting from rest accelerates uniformly at 2 m/s^2 for 5 seconds. What is the distance covered by the car in this duration?
- A. 10 m
- B. 25 m
- C. 50 m
- D. 100 m
Answer: B. 25 m — Using s = ut + (1/2)at^2, where u = 0, a = 2 m/s^2, t = 5 s. s = (0)(5) + (1/2)(2)(5^2) = 0 + 25 = 25 m.
MCQ 15. A train is moving at a speed of 90 km/h. Brakes are applied so as to produce a uniform acceleration of -0.5 m/s^2. How far will the train go before it is brought to rest?
- A. 625 m
- B. 1250 m
- C. 250 m
- D. 500 m
Answer: A. 625 m — First, convert u = 90 km/h = 25 m/s. Final velocity v = 0. Using v^2 = u^2 + 2as. 0^2 = 25^2 + 2(-0.5)s. 0 = 625 - s. So, s = 625 m.
MCQ 16. When an object is moving in a circular path with constant speed, its velocity is:
- A. Constant
- B. Always changing
- C. Zero
- D. Constant in magnitude but changing in direction
Answer: D. Constant in magnitude but changing in direction — In uniform circular motion, the object's speed (magnitude of velocity) is constant, but its direction is continuously changing. Therefore, its velocity vector is constantly changing, described as constant in magnitude but changing in direction.
MCQ 17. Which of the following is a vector quantity?
- A. Distance
- B. Time
- C. Mass
- D. Velocity
Answer: D. Velocity — Velocity is a vector quantity because it has both magnitude (speed) and direction. Distance, time, and mass are scalar quantities.
MCQ 18. What does the area under a velocity-time graph represent?
- A. Acceleration
- B. Speed
- C. Displacement
- D. Force
Answer: C. Displacement — The area under the velocity-time graph gives the displacement of the object during the given time interval.
MCQ 19. What does the slope of a distance-time graph represent?
- A. Acceleration
- B. Velocity
- C. Displacement
- D. Momentum
Answer: B. Velocity — The slope of a distance-time graph (change in distance / change in time) represents the speed or velocity of the object.
MCQ 20. What does the slope of a velocity-time graph represent?
- A. Speed
- B. Displacement
- C. Acceleration
- D. Distance
Answer: C. Acceleration — The slope of a velocity-time graph (change in velocity / change in time) represents the acceleration of the object.
How to Use These Solutions
Attempt each question yourself first and only then compare with the worked answer. Marks in the CBSE board exam are awarded for the METHOD as much as the final result, so reproduce the steps rather than memorising the last line. Where a solution states a law, a formula or a definition, learn that wording — examiners look for it.
Related practice for this chapter: chapter MCQs, previous-year questions, revision notes and sample papers.