Chapter 7: Motion Case Study Questions | science Class 9 CBSE

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Chapter 7: Motion Case Study Questions | science Class 9 CBSE | ByteNotes.in

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Case Set

Case Set 1

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While traveling by train, Meena felt like her train was moving, but then realized it was actually stationary. The illusion was created because the adjacent train started moving. Her science teacher explained the concept of motion relative to a reference point.

Question 1: Q1. What determines whether an object is in motion or at rest?

An object is in motion if it changes position with respect to a reference point.
If there is no change in position relative to the reference point, it is at rest.
Motion is always observed relative to another object or point.

Question 2: Q2. Why did Meena feel her train was moving even though it was stationary?

The adjacent train started moving, creating a relative motion effect.
Her reference point (another train) changed position.
The human brain perceives motion by comparing with surrounding objects.

Question 3: Q3. What is a reference point in the context of motion?

A reference point is a fixed position used to determine if an object has moved.
Usually considered stationary for easy comparison.
Examples include trees, buildings, or a station.

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Case Set 2

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A car travels from Bhubaneswar to Delhi. The odometer reading changes from 0 to 1850 km, but a map shows the displacement between the two cities is only 1400 km.

Question 1: Q1. What is the difference between distance and displacement?

Distance is the total path length traveled, regardless of direction.
Displacement is the shortest straight-line distance between initial and final positions.
Distance is a scalar quantity; displacement is a vector quantity.

Question 2: Q2. Why is displacement less than distance in this case?

The car did not travel in a straight line.
Roads and turns increase the actual path length.
Displacement measures only the direct separation between start and end points.

Question 3: Q3. Can distance be equal to displacement?

Yes, if the motion is in a straight line without changing direction.
For example, a car moving 100 m straight has equal distance and displacement.
But in curved or return paths, distance exceeds displacement.

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Case Set 3

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Usha swims 90 m to the other end of a pool and back in 1 minute. Her total distance is 180 m but displacement is zero. Her coach uses this to explain speed and velocity.

Question 1: Q1. What is Usha’s displacement after completing the swim?

Her displacement is zero because she returned to her starting point.
Displacement considers only the initial and final positions.
Even though she swam 180 m, start and end points are the same.

Question 2: Q2. What is the difference between speed and velocity in this example?

Speed is total distance divided by time: 180 m / 60 s = 3 m/s.
Velocity is displacement divided by time: 0 m / 60 s = 0 m/s.
Speed is scalar; velocity is vector with direction.

Question 3: Q3. Can average speed and average velocity ever be equal?

Yes, if the motion is in a straight line and no direction change occurs.
In such cases, displacement equals distance.
Otherwise, speed is usually greater than or equal to velocity.

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Case Set 4

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A driver notices that in the first 5 seconds, the car covers 25 m, in the next 5 seconds 25 m again, and so on. He concludes that the motion is uniform.

Question 1: Q1. What is uniform motion?

Motion where equal distances are covered in equal time intervals.
Speed remains constant throughout the motion.
Graphically, it gives a straight-line distance–time graph.

Question 2: Q2. What is the speed of the car in this case?

Speed = Distance / Time = 25 m / 5 s = 5 m/s.
Speed is consistent every 5 seconds.
Hence, motion is uniform with speed 5 m/s.

Question 3: Q3. What is the shape of the distance–time graph for uniform motion?

It is a straight line inclined to the time axis.
Slope of the line indicates speed.
Steeper slope means higher speed.

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Case Set 5

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A car starts from rest and accelerates uniformly. In 10 seconds it reaches a speed of 20 m/s. The driver calculates the acceleration using formulae from physics class.

Question 1: Q1. How is acceleration defined?

Acceleration is the rate of change of velocity with time.
It is given by a = (v - u) / t.
SI unit of acceleration is m/s².

Question 2: Q2. What is the acceleration of the car?

Initial velocity u = 0 m/s, final velocity v = 20 m/s, time t = 10 s.
Using a = (v - u) / t = (20 - 0) / 10 = 2 m/s².
Hence, uniform acceleration = 2 m/s².

Question 3: Q3. What does uniform acceleration mean?

Velocity changes by equal amounts in equal intervals of time.
Acceleration remains constant during motion.
Free fall and constant-speed car acceleration are examples.

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Chapter sections

Chapter 7: Motion | Case Study Questions

Passage

Question 1: Q1. What determines whether an object is in motion or at rest?

Question 2: Q2. Why did Meena feel her train was moving even though it was stationary?

Question 3: Q3. What is a reference point in the context of motion?

Passage

Question 1: Q1. What is the difference between distance and displacement?

Question 2: Q2. Why is displacement less than distance in this case?

Question 3: Q3. Can distance be equal to displacement?

Passage

Question 1: Q1. What is Usha’s displacement after completing the swim?

Question 2: Q2. What is the difference between speed and velocity in this example?

Question 3: Q3. Can average speed and average velocity ever be equal?

Passage

Question 1: Q1. What is uniform motion?

Question 2: Q2. What is the speed of the car in this case?

Question 3: Q3. What is the shape of the distance–time graph for uniform motion?

Passage

Question 1: Q1. How is acceleration defined?

Question 2: Q2. What is the acceleration of the car?

Question 3: Q3. What does uniform acceleration mean?

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