Solve all your doubts with Teachoo Black (new monthly pack available now!)

Class 9

Chapter 8 Class 9 - Motion

Last updated at May 31, 2019 by Teachoo

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·πππππ‘ππ/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 = 2πr = 2 Γ π Γ 100 = 200π m Thus, Distance covered in 40 s = 200π Distance covered in 1s = (200 π)/40 = 5π m Distance covered in 140 s = 140 Γ 5π = 700π = 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 = (πππ‘ππ π·ππ π‘ππππ πΆππ£ππππ)/(π·ππ π‘ππππ πππ£ππππ ππ 1 πππ’ππ) = 700π/200π = 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