# Class 9 Science Chapter 8 Motion

NCERT Class 9 Science Chapter 8 Motion, Question Answer to each chapter is provided in the list so that you can easily browse throughout different chapters Class 9 Science Chapter 8 Motion and select need one.

## NCERT Class 9 Science Chapter 8 Motion

Also, you can read the SCERT book online in these sections Solutions by Expert Teachers as per SCERT (CBSE) Book guidelines. These solutions are part of SCERT All Subject Solutions. Here we have given SCERT Class 9 Science Chapter 8 Motion Solutions for All Subjects, You can practice these here.

### Motion

Chapter – 8

GENERAL SCIENCE

Textbook Page No. 100

1. An object has moved through a distance. Can it have zero displacement? If yes, support your answer with an example.

Ans. Yes, the object can have zero displacement even if it has moved through a distance. For example, an object moves from a point A directly towards another point B which is at a distance of 20 m from A. The body then returns back to point A directly. As the object finally comes back to the starting position, the initial and the final position of the object are the same.

Therefore, the displacement is zero.

2. A farmer moves along the boundary of a square field of side 10 m in 40 s. What will be the magnitude of displacement of the farmer at the end of 2 minutes 20 seconds from his initial position?

Ans.

3. Which of the following is true for displacement?

(a) It cannot be zero.

Ans. False, because displacement can be zero.

(b) Its magnitude is greater than the distance travelled by the object.

Ans: False, because its magnitude is either less or equal to the distance travelled by the object.

Textbook Page No. 102.

1. Distinguish between speed and velocity.

Ans. A scalar quantity contains information about magnitude. A vector quantity, on the other hand, contains information about magnitude along with direction. An object’s speed is useful for describing how fast an object moves, while velocity involves an object’s displacement in relation to some point.

2. Under what condition (s) is the magnitude of average velocity of an object equal to its average speed?

Ans. The magnitude of average velocity of an object is equal to its average speed when the displacement of the object is equal to the distance covered by it in a given time interval.

3. What does the odometer of an automobile measure?

Ans. The odometer measures the distance travelled by the automobile.

4. What does the path of an object look like when it is in uniform motion?

Ans. The path of an object looks like a straight line when it is in uniform motion.

5. During an experiment, a signal from a spaceship reached the ground station in five minutes. What was the distance of the spaceship from the ground station? The signal travels at the speed of light, that is, 3 × 10⁸ m s⁻¹.

Ans. Speed of the signal, v = 3 × 10⁸ m s⁻¹

Time taken, t = 5 minutes = 300 s

Distance, s = ?

We know,

s = v × t

= 3 × 10⁸ m s⁻¹ × 300 s

= 9 × 10¹⁰ m

Therefore, the distance of the spaceship from the ground is 9 × 10¹⁰ m.

Textbook Page No. 103

1. When will you say a body is in

(i) Uniform acceleration?

Ans. A body moving in a straight line is said to have uniform acceleration body changes uniformly with time.

(ii) Non-uniform acceleration?

Ans. A moving body is said to have non-uniform acceleration if the velocity of the body change in the velocity body in equal intervals of time is different.

2. A bus decreases its speed from 80 km h-¹ to 60 km h-¹ in 5 s. Find the acceleration of the bus.

Therefore, the acceleration of the bus is -1.11 m s⁻².

3. A train starting from a railway station and moving with uniform acceleration attains a speed 40 km h-¹ in 10 minutes. Find its acceleration.

Textbook Page No. 107

1. What is the nature of the distance-time graphs for uniform and non-uniform motion of an object?

Ans. The distance-time graph of an object for uniform motion is a straight line and for non-uniform motion is a curved line.

2. What can you say about the motion of an object whose distance-time graph is a straight line parallel to the time axis?

Ans. If the distance-time graph of an object is a straight line parallel to the time axis, it means that the object is at rest since its position does not change with time.

3. What can you say about the motion of an object if its speed-time graph is a straight line parallel to the time axis?

Ans. If the speed-time graph of an object is a straight line parallel to the time axis, it means that the object is moving with a uniform speed.

4. What is the quantity which is measured by the area occupied below the velocity-time graph?

Ans. Distance travelled by the body in the given interval of time is measured by the area occupied below the velocity-time graph.

Textbook Page No. 109-110

1. A bus starting from rest moves with a uniform acceleration of 0.1 ms-² for 2 minutes. Find

(a) The speed acquired.

(b) The distance travelled.

Ans. Initial velocity, u = 0 m s⁻¹

Acceleration, a = 0.1 m s⁻²

Time, t = 2 minutes = 120 s

Final velocity, v = ?

Distance travelled, s = ?

2. A train is travelling at a speed of 90 km h⁻¹. Brakes are applied so as to produce a uniform acceleration of – 0.5 m s⁻². Find how far the train will go before it is brought to rest.

Therefore, the distance travelled by the train is 625 m.

3. A trolley, while going down an inclined plane, has an acceleration of 2 cm s-². What will be its velocity 3 s after the start?

Ans. Acceleration, a = 2 cm s⁻²

Initial velocity, u = 0 cm s⁻¹

Time, t = 3 s

Final velocity, v = ?

We have, v = u + at

= 0 + 2 cm s⁻² × 3 s

= 6 cm s⁻¹

= 0.06 m s⁻¹

Therefore, the velocity of the trolley will be 6 cm s-¹ or 0.06 m s⁻¹.

4. A racing car has a uniform acceleration of 4 m s⁻². What distance will it cover in 10 s after start?

Ans. Acceleration, a = 4 m s⁻²

Initial velocity, u = 0 m s⁻¹

Time, t = 10 s

Distance covered, s = ?

We have,

= 200 m

Therefore, the distance covered by the racing car is 200 m.

5. A stone is thrown in a vertically upward direction with a velocity of 5 ms⁻¹. If the acceleration of the stone during its motion is 10 ms⁻² in the downward direction, what will be the height attained by the stone and how much time will it take to reach there?

Ans. Initial velocity, u = 5 m s⁻¹

Acceleration, a = – 10 m s⁻² [As the acceleration is in the direction opposite to that of the velocity]

At the highest point,

Final velocity, v= = 0 m s⁻¹

Height attained, s = ?

Time taken, t = ?

EXERCISE

Textbook Page No. 112-113

1. An athlete completes one round of a circular track of diameter 200 m in 40 s. What will be the distance covered and the displacement at the end of 2 minutes 20 s?

Ana. As shown, O is the centre of the circular track of diameter, AB = 200 m Let A be the starting position of the athlete.

2. Joseph jogs from one end A to the other end B of a straight 300 m road in 2 minutes 30 seconds and then turns around and jogs 100 m back to point C in another 1 minute. What are Joseph’s average speeds and velocities in jogging

(a) from A to B.

(b) from A to C?

Ans. (a) The distance from A to B = 300 m

Time taken = 2 minutes 30 seconds

= 150 s

= 0.952 m s⁻¹ (approx)

3. Abdul, while driving to school, computes the average speed for his trip to be 20 km h⁻¹. On his return trip along the same route, there is less traffic and the average speed is 30 km h⁻¹. What is the average speed for Abdul’s trip?

Ans.

4. A motorboat starting from rest on a lake accelerates in a straight line at a constant rate of 3.0 ms⁻² for 8.0 s. How far does the boat travel during this time?

Ans. Here, initial velocity, u = 0

Acceleration, a = 3.0 m s⁻²

Time, t = 8.0 s

Distance, s = ?

We know,

⇒ s = 96 m

Therefore, the boat travels 96 m during this time.

5. A driver of a car travelling at 52 km h⁻¹ applies the brakes and accelerates uniformly in the opposite direction. The car stops in 5 s. Another driver going at 3 km h⁻¹ in another car applies his brakes slowly and stops in 10 s. On the same graph paper, plot the speed versus time graphs for the two cars. Which of the two cars travelled farther after the brakes were applied?

Ans.

Time taken by the first car to stop, t₁ = 5 s

Time taken by the second car to stop, t₂ = 10 s

Now, AB represents the speed-time graph of the first car and CD represents the speed- time graph of the second car.

Distance (s₁) further travelled by the first car

= Area under the speed-time graph of the first car

= Area of the ∆ ABO

= 36.11 m

Distance (s₂) further travelled by second car

= Area under the speed-time graph of the second car

= Area of the ∆ COD

= 4.167 m

Therefore, the first car travelled farther after the breaks were applied.

6. Fig. shows the distance-time graph of three objects A, B and C. Study the graph and answer the following questions:

(a) Which of the three is travelling the fastest?

(b) Are all three ever at the same point on the road?

(c) How far has C travelled when B passes A?

(d) How far has B travelled by the time it passes C?

Ans. Along the time axis (X-axis), 5 small squares represent  0.4 hours

= 4.66 km h⁻¹

Since the slope of the graph for object B is maximum, B is travelling the fastest.

(b) No, all three are never at the same point on the road.

(c) When B passes A at P, the object C is at E, that is, the distance travelled by C is ES

= 13 small squares along distance-axis

= 7.43 km

(d) When B passes C at R, the distance travelled by it is RT

= 10 small squares along distance-axis

= 5.714 km

7. A ball is gently dropped from a height of 20 m. If its velocity increases uniformly at the rate of 10 m s⁻², with what velocity will it strike the ground? After what time will it strike the ground?

Ans.

(a) Find how far does the car travel in the first 4 seconds. Shade the area on the graph that represents the distance travelled by the car during the period.

(b) Which part of the graph represents uniform motion of the car?

Ans.

(b) The straight part of the graph represents uniform motion of the car, that is, the car has a uniform motion after 6 s.

9. State which of the following situations are possible and give an example for each of these:

(a) an object with a constant acceleration but with zero velocity.

(b) an object moving in a certain direction with an acceleration in the perpendicular direction.

Ans. (a) Yes, the given situation is possible.

For example, when a stone is thrown upwards, its velocity is zero at highest point but it has acceleration equal to acceleration due to gravity, that is, 9-8 ms.

(b) Yes, the given situation is possible.

For example, a bird flying horizontally is acted upon by acceleration due to gravity acting in the vertically downwards direction, that is, in a direction perpendicular to the direction of the flying bird.

10. An artificial satellite is moving in a circular orbit of radius 42250 km. Calculate its speed if it takes 24 hours to revolve around the earth.

Ans.

Therefore, the speed of the artificial satellite is 3.07 km s⁻¹

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