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Class nine Science NCERT solutions Chap 8 Motion

Chapter 8: Motion

Intext Questions (PAGE 107)

Q.1. What is the nature of the distance-time graphs for

 (i) uniform motion

(ii) the non-uniform motion of an object?

Ans. (i) The distance-time graph for uniform motion in a straight line not parallel to the time axis.

(ii) The distance-time graph for non-uniform motion is not a straight line. It can be a curve or a zig-zag line not parallel to the time axis.

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. The object is stationary.

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. The objects are moving with a uniform motion.

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

Ans. The displacement is measured by the area under a velocity-time graph.

Intext Questions (PAGE 109)

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 speed (u) = 0 (As the bus is at rest), Final speed (ν) = ? (To be calculated)

Acceleration (a) = 0.1 ms–2. Time (t) = 2 min = 120 s

Distance travelled (s) = ? (To be calculated)

(a) Applying, ν = u + at = 0 + 0.1 ms–2 × 120 s ∴ Final speed (ν) = 12 ms–1

Distance travelled (s) = 720 m.

Q.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.

Ans. Initial speed (u) = 90 km h–1

Acceleration (a) = – 0.5 ms–2

Distance covered (s) =? (To be calculated)

Applying, ν2u2 = 2as

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

Ans. Initial velocity (u) = 0 (zero); Final velocity (ν) = ? (To be calculated)

Time (t) = 3 s

Acceleration (a) = 2 cms–2

Applying, ν = u + at = 0 + 2 cms–2 × 3 s

∴ Final velocity (ν) = 6 cms–1

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

Ans. Initial velocity (u) = 0; Acceleration (a) = 4 ms–2

Time (t) = 10 s; Distance covered (s) = ? (To be calculated)

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 of stone (u) = 5 ms–1

The final velocity of stone (ν) = 0 (stone comes to rest at some height)

Acceleration in the downward direction = 10 ms–2

∴ Acceleration in the upward direction (a) = –10 ms–2.

Height attained by stone (s) = ? (To be calculated)

Time taken to attain height (t) = ? (To be calculated)

Applying, ν = u + at ⇒  0 = 5 ms–1 – 10 ms–2 × t

Section B – Solution to Textbook Exercises