NCERT Class 9 Science Solutions: Chapter 8 – Motion Part 7 (For CBSE, ICSE, IAS, NET, NRA 2022)

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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

(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.

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