Class nine Science Chapter 8 NCERT Solution Motion

Chapter 8: Motion

Intext Questions (PAGE 100)

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

Ans. Yes, it can have zero displacements.

Example: When a person walking along a circular track returns back to the starting point, after completing one circle, his displacement is zero, but he covers a distance equal to 2πr where ‘r’ is the radius of the circular track. The displacement is zero because the shortest distance between the initial and final position of the person 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 and 20 seconds from his initial position?

Ans. Time for journey of farmer = 2 min and 20 s = 140 s.

Perimeter of square field ABCD = 4 × 10 m = 40 m.

Time for going around the square field once = 40 s.

∴ Number of times the farmer goes around the square field

Now for going once around the square field, the displacement = Zero.

Forgoing thrice around the square field, the displacement = Zero.

For ½ of the boundary of the square field, the distance covered

It is clear from the diagram that if the farmer starts from point A, then he will cover 10 m along AB and then 10 m along BC.

Displacement of a farmer from point A = Distance AC

=14.14 m

3. Which of the following is true for displacement?

(a) It cannot be zero (b) Its magnitude is greater than the distance travelled by the object.

Ans. None of the statements (a) or (b) are true for displacement.

Intext Questions (PAGE 102)

1. Distinguish between speed and velocity.

Ans. (a) Speed is the rate of change of motion, whereas velocity is the rate of change of motion in a specified direction.

(b) Speed is a scalar quantity, whereas velocity is a vector quantity.

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

Ans. When the velocity of an object changes at a uniform rate, then the magnitude of average velocity is equal to its average speed.

If ‘u’ is the initial velocity and ‘v’ is the final velocity of a body moving with uniform acceleration, then, Average speed = Average velocity.

Ans. The odometer measures the distance travelled by automobile at some particular time.

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.

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

Ans. Speed of the signal = 3 × 108 ms–1.

Time in which the signal reaches ground level = 5 min = 300 s.

∴ The distance of spaceship from the ground level = Speed × Time

= 3 × 108 ms–1 × 300 s. = 9 × 1010 m

Intext Questions (PAGE 103)

1. When will you say a body is in (i) uniform acceleration? (ii) non-uniform acceleration?

Ans. (i) When equal changes in velocity take place in equal intervals of time, then the acceleration is said to be uniform.

(ii) When unequal changes in velocity take place in equal intervals of time, then the acceleration is said to be non-uniform.

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

Ans. Initial speed (u) = 80 km h–1

Final speed (v) = 60 km h–1 =

Time = 5 s

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

Ans. The initial speed of the train (u) = Zero

The final speed of the train (ν) = 40 km h–1

Time (t) = 10 min = 600 s