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ICSE Class 7 Physics Question 9 of 23

Motion — Question 9

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9
Question

Question 4f

Select the correct alternative:

A boy goes from his house to school by bus at a speed of 20 km h-1 and returns back through the same route at a speed of 30 km h-1. The average speed of his journey is:

  1. 24 km h-1
  2. 25 km h-1
  3. 30 km h-1
  4. 20 km h-1
Answer

24 km h-1

Reason — Suppose the distance between school and house is D km.

In first part the boy goes to school by bus from home at speed (V1) = 20 km h-1.

time taken (t1) = distancespeed\dfrac{\text{distance}}{\text{speed}} =D20\dfrac{\text{D}}{\text{20}}

In second part the boy returns from school to home through same route at speed(V2) = 30 km h-1.

time taken (t2) = distancespeed\dfrac{\text{distance}}{\text{speed}} =D30\dfrac{\text{D}}{\text{30}}

Total distance = D + D = 2D km

Total time taken = D20\dfrac{\text{D}}{\text{20}} + D30\dfrac{\text{D}}{\text{30}} = 5D60\dfrac{\text{5D}}{\text{60}}

Average speed = total distance travelledtotal time taken\dfrac{\text{total distance travelled}}{\text{total time taken}}

Speed=distancetime=2D5D60=2×12=24km h−1\text{Speed} = \dfrac{\text{distance}}{\text{time}} \\[1em] = \dfrac{2D}{\dfrac{5D}{60}} \\[1em] = 2 \times 12 \\[1em] = 24 \text{km h}^{-1}