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  1. Class 9
  2. Chapter 8 Class 9 - Motion

Transcript

NCERT Question 3 Abdul, while driving to school, computes the average speed for his trip to be 20 km hโ€“1. On his return trip along the same route, there is less traffic and the average speed is 30 km hโ€“1. What is the average speed for Abdulโ€™s trip? We know that Average speed = (๐‘‡๐‘œ๐‘ก๐‘Ž๐‘™ ๐‘‘๐‘–๐‘ ๐‘ก๐‘Ž๐‘›๐‘๐‘’)/(๐‘‡๐‘œ๐‘ก๐‘Ž๐‘™ ๐‘ก๐‘–๐‘š๐‘’) Let Distance between Abdulโ€™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 = ๐‘ก_2 Home to School Distance = d Speed = 20 km/h Speed = ๐ท๐‘–๐‘ ๐‘ก๐‘Ž๐‘›๐‘๐‘’/๐‘‡๐‘–๐‘š๐‘’ 20 = ๐‘‘/๐‘‡๐‘–๐‘š๐‘’ โˆด Time = ๐‘‘/20 School to Home Distance = d Speed = 30 km/h Speed = ๐ท๐‘–๐‘ ๐‘ก๐‘Ž๐‘›๐‘๐‘’/๐‘‡๐‘–๐‘š๐‘’ 30 = ๐‘‘/๐‘‡๐‘–๐‘š๐‘’ โˆด Time = ๐‘‘/30 Total time taken = ๐‘‘/20 + ๐‘‘/30 = (3๐‘‘ + 2๐‘‘)/60 = 5๐‘‘/60 = ๐‘‘/12 Average speed = (๐‘‡๐‘œ๐‘ก๐‘Ž๐‘™ ๐‘‘๐‘–๐‘ ๐‘Ž๐‘ก๐‘›๐‘๐‘’)/(๐‘‡๐‘œ๐‘ก๐‘Ž๐‘™ ๐‘ก๐‘–๐‘š๐‘’) = 2๐‘‘/(๐‘‘/12) = (2๐‘‘ ร— 12)/๐‘‘ = 24 km/h

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CA Maninder Singh
CA Maninder Singh is a Chartered Accountant for the past 9 years and a teacher from the past 16 years. He teaches Spoken English, Written English, Grammar and Vocabulary at Englishtan.