CBSE Class 9 Science Chapter 4: Describing Motion Around Us — Assertion & Reason
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The following Assertion and Reason questions are based on Chapter 4: Describing Motion Around Us from the NCERT Class 9 Science textbook. Each question has four options - read both the Assertion (A) and Reason (R) carefully before selecting your answer.
How to attempt Assertion & Reason questions:
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is NOT the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is NOT the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Assertion & Reason Questions
1
Assertion (A): An object's state of motion can be described only after choosing a reference point.
Reason (R): An object is in motion when its position relative to the chosen reference point changes with time.
Reason (R): An object is in motion when its position relative to the chosen reference point changes with time.
Answer: (a)
Both A and R are true, and R is the correct explanation of A. Motion and rest are relative descriptions based on whether the measured position changes from the selected reference.
2
Assertion (A): Distance is a scalar quantity, whereas displacement requires both magnitude and direction.
Reason (R): The SI unit of both distance and displacement is the metre.
Reason (R): The SI unit of both distance and displacement is the metre.
Answer: (b)
Both A and R are true, but R is NOT the correct explanation of A. Both statements are correct, but sharing the same unit does not explain the scalar-vector distinction.
3
Assertion (A): A runner who completes one lap and returns to the starting point has zero displacement.
Reason (R): The total distance travelled by the runner is also zero.
Reason (R): The total distance travelled by the runner is also zero.
Answer: (c)
A is true, but R is false. The final and initial positions coincide, but the runner has covered the full length of the lap.
4
Assertion (A): The distance travelled between two positions can be smaller than the magnitude of displacement.
Reason (R): The magnitude of displacement between two positions cannot exceed the actual path length travelled.
Reason (R): The magnitude of displacement between two positions cannot exceed the actual path length travelled.
Answer: (d)
A is false, but R is true. The straight-line separation is the shortest possible path, so distance is always at least as large as displacement magnitude.
5
Assertion (A): Average speed equals total distance travelled divided by the total time interval.
Reason (R): Average speed accounts for the complete path covered during the measured interval.
Reason (R): Average speed accounts for the complete path covered during the measured interval.
Answer: (a)
Both A and R are true, and R is the correct explanation of A. Using the entire distance and entire time gives the average rate of path coverage.
6
Assertion (A): Average velocity equals displacement divided by the corresponding time interval.
Reason (R): Average speed equals total distance divided by the total time interval.
Reason (R): Average speed equals total distance divided by the total time interval.
Answer: (b)
Both A and R are true, but R is NOT the correct explanation of A. Both definitions are correct, but the average-speed definition does not explain average velocity.
7
Assertion (A): An object moving with constant velocity in a straight line has zero acceleration.
Reason (R): Acceleration depends only on the magnitude of velocity and not on its direction.
Reason (R): Acceleration depends only on the magnitude of velocity and not on its direction.
Answer: (c)
A is true, but R is false. Acceleration is the rate of change of the velocity vector, including both magnitude and direction.
8
Assertion (A): An object moving in a circle at constant speed has zero acceleration.
Reason (R): Its velocity changes continuously because the direction of motion changes at every point.
Reason (R): Its velocity changes continuously because the direction of motion changes at every point.
Answer: (d)
A is false, but R is true. A direction change is a velocity change, so uniform circular motion is accelerated motion.
9
Assertion (A): The slope of a position-time graph represents velocity.
Reason (R): Slope measures the change in position per unit change in time.
Reason (R): Slope measures the change in position per unit change in time.
Answer: (a)
Both A and R are true, and R is the correct explanation of A. This ratio is precisely the definition of velocity along the chosen line.
10
Assertion (A): A horizontal line on a position-time graph represents an object at rest.
Reason (R): The area under a velocity-time graph gives displacement over the selected interval.
Reason (R): The area under a velocity-time graph gives displacement over the selected interval.
Answer: (b)
Both A and R are true, but R is NOT the correct explanation of A. Both graphical statements are correct, but the area rule does not explain the horizontal position-time line.
11
Assertion (A): A straight position-time graph with constant non-zero slope represents uniform velocity.
Reason (R): A less steep position-time graph always represents a greater velocity magnitude.
Reason (R): A less steep position-time graph always represents a greater velocity magnitude.
Answer: (c)
A is true, but R is false. For the same graph scales, a steeper slope has the greater velocity magnitude.
12
Assertion (A): The area under a position-time graph directly gives the distance travelled.
Reason (R): The signed area under a velocity-time graph gives displacement.
Reason (R): The signed area under a velocity-time graph gives displacement.
Answer: (d)
A is false, but R is true. Position-time slope, rather than area, provides velocity; area under velocity-time gives displacement.
13
Assertion (A): Average acceleration is the change in velocity divided by the time interval.
Reason (R): A change in either speed or direction produces a change in velocity.
Reason (R): A change in either speed or direction produces a change in velocity.
Answer: (a)
Both A and R are true, and R is the correct explanation of A. Because velocity is a vector, both types of change contribute to acceleration.
14
Assertion (A): Acceleration may be negative with respect to a chosen positive direction.
Reason (R): An object can have negative velocity while its acceleration is zero.
Reason (R): An object can have negative velocity while its acceleration is zero.
Answer: (b)
Both A and R are true, but R is NOT the correct explanation of A. Both statements are possible, but the second is an example and does not explain the meaning of negative acceleration.
15
Assertion (A): A horizontal line above the time axis on a velocity-time graph represents constant non-zero velocity.
Reason (R): Such a graph necessarily represents zero displacement over every time interval.
Reason (R): Such a graph necessarily represents zero displacement over every time interval.
Answer: (c)
A is true, but R is false. A non-zero constant velocity produces non-zero displacement that increases with time.
16
Assertion (A): A speedometer reading that remains constant proves that a vehicle has zero acceleration.
Reason (R): A vehicle turning at constant speed accelerates because its velocity direction changes.
Reason (R): A vehicle turning at constant speed accelerates because its velocity direction changes.
Answer: (d)
A is false, but R is true. A speedometer shows only speed, so it cannot rule out acceleration caused by turning.
17
Assertion (A): For constant acceleration, final velocity is related to initial velocity by v = u + at.
Reason (R): Constant acceleration produces equal changes in velocity during equal time intervals.
Reason (R): Constant acceleration produces equal changes in velocity during equal time intervals.
Answer: (a)
Both A and R are true, and R is the correct explanation of A. Accumulating the same velocity change a during each unit of time yields the stated relation.
18
Assertion (A): The standard kinematic equations apply to straight-line motion with constant acceleration.
Reason (R): Uniform circular motion is motion along a circular path with constant speed.
Reason (R): Uniform circular motion is motion along a circular path with constant speed.
Answer: (b)
Both A and R are true, but R is NOT the correct explanation of A. Both statements are correct, but the circular-motion definition does not explain the condition for using straight-line kinematic equations.
19
Assertion (A): In uniform circular motion, acceleration is directed toward the centre of the circular path.
Reason (R): The instantaneous velocity is also directed toward the centre.
Reason (R): The instantaneous velocity is also directed toward the centre.
Answer: (c)
A is true, but R is false. Instantaneous velocity is tangential to the path, while centripetal acceleration points inward.
20
Assertion (A): Distance and displacement are always equal for any journey.
Reason (R): Their magnitudes are equal only when motion follows a straight path without reversing direction.
Reason (R): Their magnitudes are equal only when motion follows a straight path without reversing direction.
Answer: (d)
A is false, but R is true. Changes of direction increase path length without producing an equal increase in net displacement.
❓ Frequently Asked Questions
What is covered in CBSE Class 9 Science Chapter 4 Describing Motion Around Us?
This chapter covers all key topics from Describing Motion Around Us as per CBSE 2026-27 syllabus.
Is this Assertion & Reason useful for CBSE board exams?
Yes, designed for CBSE Class 9 board exam preparation covering the complete syllabus.
Are these CBSE Class 9 Science Assertion & Reason updated for 2026-27?
Yes, all content at eBookPublisher is updated as per the latest 2026-27 CBSE syllabus.
How many chapters are in CBSE Class 9 Science?
All chapters of CBSE Class 9 Science are covered at eBookPublisher with free Assertion & Reason for each chapter.
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