NCERT Class 9 Science Solutions: Chapter 8 – Motion Part 6

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Question 3:

Q. Abdul, while driving to school, computes the average speed for his trip to be 20 km. On his return trip along the same route, there is less traffic and the average speed is 40 km. What is the average speed for abdul’s trip?


Case I: While driving to school

Average speed of Abdul’s trip

Total distance = Distance travelled to reach school=d

Case II: While returning from school

Total distance = distance travelled while returning from school = d

Now, total time taken

Average speed for Abdul’s trip


Total distance covered in the trip

Total time taken,

From equations (i) and (ii),

Hence, the average speed for Abdul’s trip is 26.67 m/s.

Question 4:

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


Initial velocity,(since the motor boat is initially at rest)

Acceleration of the motorboat,

Time taken, s

According to the second equation of motion:

Distance covered by the motorboat, s

Hence, the boat travels a distance of 96m.

Question 5

Q. A driver of a car travelling at 52 km applies the brakes and accelerates uniformly in the opposite direction. The car stops in 5 s. another driver going at 3 km in another car applies his brakes slowly and stop 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?


Case A:

Initial speed of the car,

Time taken to stop the car, s

Final speed of the car becomes zero after 5 s of application of brakes.

Case B:

Initial speed of the car,

Time taken to stop the car,

Final speed of the car becomes zero after 10 s application of brakes.

Plot of the two cars on a speed-time graph is shown in the following figure:

Speed Time Graph

Distance covered by each car is equal to the area under the speed-time graph.

Distance covered in case A,

Distance covered in case B,

Area of

Thus, the distance covered in case A is greater than the distance covered in case B.

Hence, the car travelling with a speed of 52 km/h travels farther after brakes were applied.

Q 5 Graph Showing Time V/S Speed.

Q 5 Graph Showing Time V/S Speed

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