Class 9 · Advanced ScienceNCERT 2026 Revised Syllabus
Chapter 2: Understanding Motion through Experience
Science Advanced
Rest and motion, frame of reference, scalar and vector quantities, distance vs displacement, vector addition and subtraction, and distance covered in the nth second.
Find complete NCERT solutions for Class 9 Advanced Science, Science Advanced Chapter 2. Every question has a clear, step-by-step answer.
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Reflect (Chapter Opening Questions)(4 questions)
1
Why do we feel pushed backward when a bus suddenly starts moving?
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Ans.
Solution:- Due to inertia, our body tends to stay at rest while the bus moves forward.
- Relative to the bus, we feel pushed backward.Answer: Due to inertia, our body tends to stay at rest while the bus moves forward, so relative to the bus we feel pushed backward.
2
Can an object be at rest for one observer but moving for another?
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Ans.
Solution:- Yes. Motion is relative.
- An object at rest in one frame of reference can appear to be moving when viewed from a different frame.Answer: Yes. Motion is relative; an object at rest in one frame of reference can appear to be moving when viewed from a different frame.
3
How do athletes decide the best angle to throw a ball so that it travels the maximum distance?
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Ans.
Solution:- They apply projectile motion principles.
- On level ground with no air resistance, a launch angle of 45 degrees gives the maximum horizontal range.Answer: A launch angle of 45 degrees gives the maximum horizontal range.
4
Can we measure motion using simple tools available in the classroom?
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Ans.
Solution:- Yes. Using a measuring tape and a stopwatch.
- Record distance and time to calculate the speed of a moving object.Answer: Yes, using a measuring tape and a stopwatch, distance and time can be recorded to calculate the speed of a moving object.
Practice Question (Sita’s Journey)(1 questions)
1
Hard
Points A(1,1), B(3,1), C(3,5), D(4,5) in km represent Sita’s house, bus stop, traffic signal and school. She walks A to B, then travels B to D via C by bus. Calculate: (a) distance on foot (b) distance by bus (c) total displacement from house to school.
- a.(a) distance on foot
- b.(b) distance by bus
- c.(c) total displacement from house to school
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Ans.
Solution:(a) Distance on foot (A to B) =
$2 \text{ km}$.(b) Distance by bus (B to C to D) = BC + CD = $4 + 1 = 5 \text{ km}$.(c) Displacement (A to D, straight line) = $\sqrt{3^{2} + 4^{2}} = \sqrt{25} = 5 \text{ km}$.Answer: (a) 2 km. (b) 5 km. (c) 5 km.Reflect and Discuss(3 questions)
1
Why is specifying a reference frame necessary to describe motion?
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Ans.
Solution:- Motion is relative.
- Without a fixed reference point, we cannot determine whether an object's position is actually changing or not.Answer: Motion is relative; without a fixed reference point, we cannot determine whether an object's position is actually changing or not.
2
How do direction and magnitude together describe displacement?
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Ans.
Solution:- Displacement is a vector quantity.
- Its magnitude gives the shortest distance between initial and final points.
- Its direction shows the path's orientation.Answer: Displacement is a vector quantity; its magnitude gives the shortest distance between initial and final points, while direction shows the path's orientation.
3
Which daily activities around you involve accelerated motion?
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Ans.
Solution:- A bus starting or stopping.
- A ball falling under gravity.
- A cyclist speeding up.
- A car taking a turn at changing speed.Answer: A bus starting or stopping, a ball falling under gravity, a cyclist speeding up, and a car taking a turn at changing speed.
Project-Based Learning(1 questions)
1
Design a simple experiment using everyday materials to measure the speed of a moving object (bicycle or walking student).
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Ans.
Solution:- Mark two points a known distance apart using chalk.
- Measure the time taken by the moving object to cover this distance with a stopwatch.
- Calculate speed = distance ÷ time, and repeat for accuracy.Answer: Mark two points a known distance apart; measure the time with a stopwatch; calculate speed as distance divided by time, and repeat for accuracy.
Check Your Understanding(11 questions)
1
Define a frame of reference in your own words.
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Ans.
Solution:A frame of reference is a fixed point or system relative to which the position and motion of an object are described and measured.Answer: A frame of reference is a fixed point or system relative to which the position and motion of an object are described and measured.
2
Give two real-life examples where motion depends on the observer.
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Ans.
Solution:- A co-passenger appears at rest to another passenger moving with the same speed in a train.
- Trees appear to move backward to someone inside a moving car.Answer: A co-passenger appears at rest to a passenger moving with the same speed in a train; trees appear to move backward to someone inside a moving car.
3
Why does a person sitting in a moving train appear at rest to another passenger?
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Ans.
Solution:- Both passengers move with the same velocity.
- Their relative position does not change.
- So each appears stationary with respect to the other.Answer: Since both passengers move with the same velocity, their relative position does not change, so each appears stationary with respect to the other.
4
Classify the following as scalar or vector: speed, velocity, displacement, distance, acceleration, mass.
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Solution:- Scalars: speed, distance, mass.
- Vectors: velocity, displacement, acceleration.Answer: Scalars - speed, distance, mass; Vectors - velocity, displacement, acceleration.
5
Explain the difference between distance and displacement with an activity diagram.
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Solution:- Distance is the total path length travelled (scalar).
- Displacement is the shortest straight-line distance between start and end points (vector).
- Walking from A to B and back to A gives distance 2AB but zero displacement.Answer: Distance is the total path length travelled (scalar); displacement is the shortest straight-line distance between start and end points (vector).
6
Give two everyday examples of vector quantities.
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Ans.
Solution:- The displacement of a car travelling from home to school.
- The velocity of wind blowing in a specific direction.Answer: The displacement of a car travelling from home to school, and the velocity of wind blowing in a specific direction.
7
Hard
Draw two vectors of 4 units east and 3 units north and find the resultant using the triangle method.
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Ans.
Solution:- Place the two vectors head to tail.
- The resultant joins the start point to the final point.
- Magnitude
$= \sqrt{4^{2} + 3^{2}} = \sqrt{25} = 5$ units, directed north-east of the start.Answer: Resultant magnitude = 5 units, directed north-east of the start.8
Explain how vector subtraction is performed graphically.
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Solution:- To subtract vector B from vector A, reverse the direction of B to get -B.
- Then add A and -B head to tail using the triangle method.
- The resulting line gives A minus B.Answer: Reverse the direction of B to get -B, then add A and -B head to tail; the resulting line gives A minus B.
9
Draw two opposite vectors of equal magnitude. Calculate its resultant.
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Ans.
Solution:- Two vectors of equal magnitude acting in exactly opposite directions.
- They cancel each other completely.
- Resultant = zero magnitude.Answer: Two vectors of equal magnitude acting in exactly opposite directions cancel each other completely, giving a resultant of zero magnitude.
10
Hard
A body starts from rest and accelerates at 4 m/s2. Find the distance travelled in the 6th second.
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Solution:
$s(n) = u + \frac{a}{2}(2n - 1)$$s(6) = 0 + \frac{4}{2}(2 \times 6 - 1) = 2 \times 11 = 22 \text{ m}$Answer: Distance travelled in the 6th second = 22 m.11
Hard
A car with initial velocity 8 m/s accelerates at 2 m/s2. Find the distance covered in the 5th second.
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Ans.
Solution:
$s(n) = u + \frac{a}{2}(2n - 1)$$s(5) = 8 + \frac{2}{2}(2 \times 5 - 1) = 8 + 9 = 17 \text{ m}$Answer: Distance covered in the 5th second = 17 m.Activities — How to Do(3 questions)
1
Activity 2.1 (Let us observe): walk and run the same fixed distance and compare the time taken. Which takes longer, and what factors can change the result?
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Ans.
Time both over the same marked distance with a stopwatch — walking takes longer because each stride is slower and shorter. Timings vary with fitness, slope, surface, footwear and load. Repeating twice and averaging gives a fairer comparison.
2
Which motion is faster — walking or running? How can you calculate speed?
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Ans.
Running is faster — it covers the same distance in less time. Speed = distance ÷ time: e.g., 50 m in 25 s (walk) = 2 m/s vs 50 m in 8 s (run) = 6.25 m/s. Use metres–seconds (m/s) or kilometres–hours (km/h); average speed uses total distance ÷ total time.
3
Can motion be described using measurable quantities such as distance and time?
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Ans.
Yes. Distance and time are directly measurable (tape, stopwatch); from them come speed, velocity, displacement and acceleration. These quantities let us compare motions, predict arrivals and test claims — the start of describing motion scientifically.