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NCERT Question 3 - Chapter 8 Class 9 - Motion (Term 1)

Last updated at June 6, 2019 by

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Transcript

NCERT Question 3 Abdul, while driving to school, computes the average speed for his trip to be 20 km h–1. On his return trip along the same route, there is less traffic and the average speed is 30 km h–1. What is the average speed for Abdul’s trip? We know that Average speed = (π‘‡π‘œπ‘‘π‘Žπ‘™ π‘‘π‘–π‘ π‘‘π‘Žπ‘›π‘π‘’)/(π‘‡π‘œπ‘‘π‘Žπ‘™ π‘‘π‘–π‘šπ‘’) Let Distance between Abdul’s Home and School be d Now, Since he goes from home to school, and then back Total Distance = d + d = 2d To find Total Time, we find Time taken to go from Home to School, and from School to Home Let time taken to reach home = 𝑑_2 Home to School Distance = d Speed = 20 km/h Speed = π·π‘–π‘ π‘‘π‘Žπ‘›π‘π‘’/π‘‡π‘–π‘šπ‘’ 20 = 𝑑/π‘‡π‘–π‘šπ‘’ ∴ Time = 𝑑/20 School to Home Distance = d Speed = 30 km/h Speed = π·π‘–π‘ π‘‘π‘Žπ‘›π‘π‘’/π‘‡π‘–π‘šπ‘’ 30 = 𝑑/π‘‡π‘–π‘šπ‘’ ∴ Time = 𝑑/30 Total time taken = 𝑑/20 + 𝑑/30 = (3𝑑 + 2𝑑)/60 = 5𝑑/60 = 𝑑/12 Average speed = (π‘‡π‘œπ‘‘π‘Žπ‘™ π‘‘π‘–π‘ π‘Žπ‘‘π‘›π‘π‘’)/(π‘‡π‘œπ‘‘π‘Žπ‘™ π‘‘π‘–π‘šπ‘’) = 2𝑑/(𝑑/12) = (2𝑑 Γ— 12)/𝑑 = 24 km/h

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CA Maninder Singh is a Chartered Accountant for the past 12 years and a teacher from the past 16 years. He teaches Science, Accounts and English at Teachoo