NCERT Class 9 Science Solutions: Chapter 8 – Motion Part 7

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Question 6

Q. Fig 8.11 shows the distance-time graph of three objects A, B and C. Study the Graph and answer the following question:

Distance-Time Graph of Three Objects a, B and C

(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?

(a) Object B

(b) No

(c) 5.714 km

(d) 5.143 km

Answer:

Q 6 Image Showing distance-time graph.

Q 6 Image Showing Distance-Time Graph

Loading image

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

Distance-Time Graph of Three Objects a, B and C

Since slope of object B is greater than objects A and C, it is travelling the fastest.

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

All three objects A, B and c never meet at a single point. Thus, they were never at the same point on road.

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

Distance-Time Graph of Three Objects a, B and C

On the distance axis:

7 small boxes = 4km

Initially, object C is 4 blocks away from the origin.

Intial distance of object C from origin

Distance of object C from origin when B passes

Distance covered by

Hence, C has travelled a distance of 5.714 km when B passes A.

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

Distance-Time Graph of Three Objects a, B and C

Distance covered by B at the time it passes boxes

Hence, B has travelled a distance of 5.143 km when B passes A.

Question 7

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

Answer:

Distance covered by the ball,

Acceleration,

Initially, velocity, (since the ball was initially at rest)

Final velocity of the ball with which it strikes the ground, v

According to the third equation of motion:

According to the first equation of motion:

Where,

Time, t taken by the ball to strike the ground is,

Hence, the ball strikes the ground after 2 s with a velocity of 20 m/s.