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

Transcript

NCERT Question 1 An athlete completes one round of a circular track of diameter 200 m in 40 s. What will be the distance covered and the displacement at the end of 2 minutes 20 s? Diameter of the track = 200 m Radius of track = π·π‘–π‘Žπ‘šπ‘’π‘‘π‘’π‘Ÿ/2 = 200/2 = 100 m Finding Distance Now, Time taken to cover 1 round = 40 s Total time the athlete moves = 2 min 20 sec = 2 Γ— 60 + 20 = 140 s Distance covered in 1 round = Circumference of circle = 2πœ‹r = 2 Γ— πœ‹ Γ— 100 = 200πœ‹ m Thus, Distance covered in 40 s = 200πœ‹ Distance covered in 1s = (200 πœ‹)/40 = 5πœ‹ m Distance covered in 140 s = 140 Γ— 5πœ‹ = 700πœ‹ = 700 Γ— 22/7 = 100 Γ— 22 = 2200 m Finding Displacement Now, it is given that it takes 40 seconds to cover 1 round and he runs for 220 seconds We find Total Number of Rounds Total rounds = (π‘‡π‘œπ‘‘π‘Žπ‘™ π·π‘–π‘ π‘‘π‘Žπ‘›π‘π‘’ πΆπ‘œπ‘£π‘’π‘Ÿπ‘’π‘‘)/(π·π‘–π‘ π‘‘π‘Žπ‘›π‘π‘’ π‘π‘œπ‘£π‘’π‘Ÿπ‘’π‘‘ 𝑖𝑛 1 π‘Ÿπ‘œπ‘’π‘›π‘‘) = 700πœ‹/200πœ‹ = 7/2 = 3.5 rounds Therefore, If he started from A, After140 s, he will be at B Thus, Displacement = Straight line from A to B = Diameter of the circle = 200 m

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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.